The Noble Quran and Prophet Muhammad specifically refer to the leadership of Imam Ali after the Prophet in several incidents.
This incident took place on the 18th of Dhul Hijjah, the twelfth month of theu Islamic calendar, and has been narrated by 110 companions of the Prophet, 84 members of the following generation (the tabi΄in), and 360 Muslim scholars from all schools of thought. Prophet Muhammad and approximately 114,000 of his companions had performed the farewell Hajj (pilgrimage) and were returning home. That year, during the Hajj, the weather was very hot with the blazing sun taking its toll on the pilgrims. When the Prophet arrived at Ghadir Khum, a marshland crossroads from which all the Muslims from different lands would part on their own ways, the Prophet stopped the caravan at noontime, and waited for those who were behind to arrive and called upon those who had gone ahead to return, for he had received a revelation from Allah which he had to deliver to the people. The revelation read, “O Messenger! Declare what has been revealed to you from Your Lord, and if you do not, then your mission will not have been fully declared, and Allah will protect you from the harm of the people.” Then the Prophet spoke a bit before asking the assembly whether he truly had authority over them or not. The people replied, “Yes, O Prophet, of course you are our leader (mawla).” He repeated this question three times, and the people responded in the same way each time, acknowledging his leadership. The Prophet then called for Ali, held up his arm so that their two arms formed one shape pointing upwards, and said to the people, “He whose leader (mawla) I am, Ali is his leader.”
At that time, Ali was 33 years old. The people received this news with a variety of responses—some with happiness and some with resentment. The foremost to congratulate Ali were the future first and second caliphs; the second caliph said, “Congratulations, congratulations to you, O Ali; you have become my leader (mawla) and the leader of every faithful Muslim.”
After declaring the mentioned revelation another verse was revealed to Prophet Muhammad. Allah said, “Today I have completed for you the religion, and favored you with My bounty, and accepted Islam for you as the religion.” With this verse, the religion of Islam was completed by the appointment of Imam Ali to succeed the Prophet, and had he not been appointed as the successor, the religion of Islam would have been incomplete as is specifically mentioned in these verses.
The Verse of Warning (Indhar)
Three years after the advent of Islam, Allah commanded the Prophet to proclaim his invitation to Islam to his immediate family in Makkah by commanding, “And warn your tribe who are of near kindred.” The Prophet gathered forty members of his tribe, Bani Hashim and held a feast inside the house of his uncle Abu Talib by preparing food for them. After they had finished eating, the Prophet said to them, “O children of 'Abd al-Mutallib! By Allah, I don't know any young person from among the Arabs who has brought his people something better than that which I am bringing you. I have brought you the best of this world and the next, and Allah has commanded me to invite you to it. So who will be my supporter in this endeavor, to be my brother, my successor (khalifah), and legatee?” No one stood up to accept this invitation except Ali ibn Abi Talib, who was only about 13 years old at the time, said, “I will be your supporter in this endeavor.” The Prophet requested him to sit down and then repeated his question a second time. Again, only Ali stood up, and again the Prophet asked him to sit. When even the third time the Prophet heard no answer from the other family members, Ali stood up again and repeated his support. The Prophet then put his hand on his leg and said to the forty men from his immediate family, “This is my brother, my legatee, and my successor (khalifah) over you, so listen to him and obey him.” The men stood and while laughing told the father of Ali, “Your nephew has ordered you to listen to your son and obey him.”
The Verse of Bowing (Ruku)
Verily, your guardian (wali) is Allah, His messenger, and the believers; those who perform the prayers and give zakat (alms) while bowing down (in ruku).9
Numerous commentators of the Quran from all schools of thought identify the one referred to in this verse is Ali ibn Abi Talib. The famous commentator, Zamakhshari says about this verse, “It was revealed in favor of Ali (may Allah enlighten his face). When a beggar asked him for alms in the masjid and Ali was in the position of ruku during the prayers, he gave away his ring while in that position. It seems it was loose on the little finger, for he did not exert any effort in taking it off, which would have nullified his prayer. If you ask how it could be in favor of Ali since the wording is in the plural form, I would say that the form is plural although its instigator is a single man to encourage people to follow his example and earn a similar reward; and also to draw attention to the fact that the believers must be extremely mindful and benevolent towards the poor such that if a situation can not be postponed until after the prayer, then it should not be delayed until having finished it.”)
Similarly, al-Wahidi in his book on the commentary of the Quran entitled, Asbab al-Nuzul, cites Kalbi's narration, that the cause of this revelation was Imam Ali. Kalbi says, “The later part of this verse is in favor of Ali ibn Abi Talib (may Allah be gracious to him) because he gave his ring to a beggar while in the state of ruku during the prayers.”) Many other commentaries also hold that this verse refers to Imam Ali including: Sunan al-Nisa΄i, Tafsir al-Kabir by Tha΄alibi, Musnad Ahmad ibn Hanbal, Musnad ibn Marduwayh, and Kanz al-'Ummal.
The Verse of Guardianship
O you who believe! Obey Allah, and obey the Messenger, and those vested with authority over you ('Ul ul-'amr minkum). And if you quarrel about something, then refer it to Allah and the Messenger.10
By the explanation from the Prophet Muhammad, this verse is also one of the Quranic references to the leadership of Imam Ali after the Prophet, and it necessitates the obedience of the faithful to Allah, the Prophet, and those vested with authority over them. When this verse was revealed to the Prophet, one of his great companions, Jabir ibn 'Abdullah al-Ansari asked:
O Prophet of Allah! We know Allah and His Messenger, but who are 'those vested with authority over you' ('Ul ul-'amr) whose obedience Allah considers equal to that of Allah and the Prophet? The Prophet replied that those are my successors and the leaders of the Muslims after me. The first of them is Ali ibn Abi Talib, then al-Hassan and al-Husayn, then Ali ibn al-Husayn, then Muhammad ibn Ali, who is known as al-Baqir. You, Jabir will see him and when you see him, give him my salam. Then al-Sadiq Ja'far ibn Muhammad, then Musa ibn Ja'far, then Ali ibn Musa, then Muhammad ibn Ali, then Ali ibn Muhammad, then al-Hassan ibn Ali, then the one who bears my name, Muhammad. And he will be the proof (hujjah) of Allah on the earth.
Prophetic Narrations Appointing Imam Ali as Successor
Prophet Muhammad told the Muslims both about the succession of the designated members of his family (Ahlul Bayt), which will be dealt with in the next section, as well as the specific succession of Imam Ali. The Messenger of Allah has been recorded to have said in regards to Imam Ali:
He who wishes to live as I have lived and to die as I will die, and enter the Garden of Eternal Bliss which Allah has promised me—let him take Ali as his leader (wali), because Ali will never lead you away from the Path of Truth, nor will he take you into error.
At the Battle of Khaybar the Muslims were struggling to conquer the castle. The two companions, Abu Bakr and Umar had previously failed in their attempts to defeat the enemies, at which the Messenger of Allah said, I will certainly give this standard (i.e. flag) to a man whom Allah and His Messenger love.” Other narrations say that the Prophet said, “Allah will grant victory through the one who loves Allah and His Messenger.” In either case, the Prophet Muhammad gave the standard to Ali, and Allah granted victory through his hand.
Twelve Leaders to Succeed the Prophet
In addition to the specific narrations emphatically identifying Imam Ali as the successor of the Prophet Muhammad, the Prophet was also recorded to have said on numerous occasions that after him he would be succeeded by twelve leaders from his tribe of Quraysh. Some of these narrations are:
I joined the company of the Prophet with my father and heard him say, “This caliphate will not end until there have come the twelve caliphs among them.” The narrator said, “Then he (the Prophet) said something which I could not follow.” I said to my father, “What did he say?” He said, “He has said, 'all of them will be from Quraysh.”'
Nevertheless, extensive research show that the accurate version of the Prophet's narration is that 'all of them will be from Bani Hashim,' which is exclusive to the imams of Ahlul Bayt.
Who are the Twelve Leaders?
The Prophet said:
I and Ali are the fathers of this nation. He who recognizes us as such believes in Allah, the Mighty and Glorious. And from Ali are my two grandchildren, Hassan and Husayn, each of whom is a prince over the youth in Heaven; and among the descendants of Husayn are nine. Obedience to them is obedience to me, and disobedience to them is disobedience to me. The ninth of them is the Qa΄im (the firmly established) and Mahdi—the executor and the one divinely trained for right guidance.
Stated to his grandson Husayn when he was only a few years old, the Prophet said to him:
You are a sayyid (master) and the son of a sayyid. You are an imam and the son of an imam, the brother of an imam and the father of imams. You are Allah's proof and confirmation and the son of His proof. You are the father of nine of Allah's proofs in your line of descendants. The ninth of them is the Qa΄im (the firmly established, the executor).
 Noble Quran, 5:67. See the following commentators (mufassirin): Tabari, Wahidi, Tha‘alibi, Qurtubi, al-Razi, Ibn Kathir, Naysaburi, Suyuti, and Alusi al-Baghdadi, and the following historians: Balathari, Ibn Qutaybah, Tabari, al-Khatib al-Baghdadi, Ibn ‘Abd al-Birr, Shahristani, Ibn Asakir, Ibn al-Athir, Ibn Abi al-Hadid, Ya‘qut al-Hamawi, Ibn Khalaqan, Yafi‘i, Ibn Kathir, Ibn Khuldun, al-Dhahabi, Ibn Hajar al-Askalani, Ibn al-Sabbagh al-Maliki, al-Maqrizi, and Jalal al-Din al-Suyuti, and also the following recorders of hadith: al-Shafi‘i, Ahmad ibn Hanbal, Ibn Majah, Tirmidhi, Nisa΄i, al-Baghawi, al-Dulabi, al-Tahawi, Abu Ya΄la al-Musali, al-Hakim al-Naysaburi, Khatib al-Khawarizmi, Muhibb al-Din al-Tabari, al-Dhahabi, and al-Muttaqi al-Hindi.
 Ahmad ibn Hanbal, Musnad, Vol. 4, 281; al-Ghazali, Sirr al-‘Alamin, 12; al-Tabari, al-Riyadh al-Nadhirah, Vol. 2, 169
 Noble Quran, 5.3.
 Noble Quran, 26:214
 Ihqaq al-Haqq, Vol. 4, 62; Tarikh al-Tabari, Vol. 2, 117; Musnad Ahmad ibn Hanbal, Vol. 1, 159; Tarikh Abul Fida, Vol. 1, 116; Nadhm Durar al-Simtayn, 82; Kifayat al-Talib, 205; Tarikh Madinat Dimishq, Vol.1, Hadith 87, 139 and 143; al-Hasakani, Shawahid al-Tanzil, Vol. 1, 420; Muhammad ibn Jarir al-Tabari, Jami‘ al-Bayan, Vol. 19, 131; Jalal al-Din al-Suyuti, al-Durr al-Manthur, Vol. 5, 97; Tafsir ibn Kathir, Vol. 3, 350; al-Baghdadi, Tafsir al-Khazin, Vol. 3, 371; al-Alusi al-Baghdadi, Ruh al-Ma΄ani, Vol. 19, 122; al-Tantawi, Tafsir al-Jawahir, Vol. 13, 103; al-Hakim al-Naysaburi, al-Mustadrak ‘ala al-Sahihayn, Vol. 3, 135. Other historical sources, such as Sirat al-Halabi, say that the Prophet added, “And he will be my minister (wazir) and inheritor (warith).”
 Zamakhshari, Tafsir al-Kashif (See interpretation of ch. 5 v. 55
 Wahidi, Asbab al-Nuzul, (See interpretation of ch. 5 v. 55
 Noble Quran, 5:38
 Vol. 6, Hadith 391 and 5991
 i.e. peaceful greeting or greetings of peace.
 Sahih al-Bukhari, “Book on Outstanding Traits” Hadith 3430, “Battles” Hadith 4064; Sahih Muslim, “Book of the Merits of the Companions” Hadith 4418; al-Tirmidhi, “Book on Outstanding Traits” Hadith 3664; Ibn Majah, “Book on the Introduction” 112 and 118; Musnad Ahmad ibn Hanbal, Vol.1, 173, 175, 177, 179, 182, 184, and 185.
 al-Hakim, al-Mustadrak, Vol. 3, 128; al-Muttaqi al-Hindi, Kanz al-Ummal, Vol. 6, 155
 Musnad Ahmad ibn Hanbal, Vol. 5, 25; Sahih Tirmidhi, Vol. 5, 296
 Kanz al-Ummal, al-Muttaqi al-Hindi, Vol. 6, 170
 al-Hakim, al-Mustadrak, Vol. 3, 226; Ibn Jarir, Kanz al-Ummal; al-Muttaqi al-Hindi, Vol. 15, 13; Tarikh ibn Kathir, Vol. 7, 358
 Sunan ibn Majah, Vol. 1, 44; Sahih Tirmidhi, Vol. 5, 300
 al-Hakim, Vol. 3, 221, al-Dhahabi
 Sahih al-Bukhari, “Book of Jihad and Marching” Hadith 2724 and 2753, “Battles” Hadith 3888; Sahih Muslim, “Book on the Merits of the Companions” Hadith 4423-4424; Musnad Ahmad ibn Hanbal, Vol. 5, 333
 Sahih al-Bukhari, “Book on Outstanding Traits”, Hadith 3240; Sahih Muslim, Kitab al-Imarah, Hadith 3392; Musnad Ahmad ibn Hanbal, Part 2. 29, 93, and 128
 Sahih al-Bukhari, “Book on Legal Judgments” Hadith 6682; Sahih Muslim, Kitab al-Imarah, Hadith 3393; al-Tirmidhi, “Book on the Trials” Hadith 2149; Abu Dawud, “Book on al-Mahdi” Hadith 3731; Musnad Ahmad ibn Hanbal, Vol. 5, 87, 90, 92, 95, 97, 99-101, and 106-108
 Ikmal al-Din