Imam Ja'far al-Sadiq
Jaʿfar ibn Muḥammad (Arabic: جعفر بن محمد الصادق; 700 or 702–765 C.E.), commonly known as Jafar al-Sadiq or simply as-Sadiq (The Truthful), was the sixth Shī'ah Imam, and a major figure in the Hanafi and Maliki schools of Sunni jurisprudence. He was a descendant of Ali on the side of his father, Muhammad al-Baqir, and of Muhammad ibn Abu Bakr on the maternal side of his family, Umm Farwah bint al-Qasim. Muhammad ibn Abu Bakr was raised by Ali, but was not his son.
Our scholars study the chain of narrators and the text of every narration to assess the authenticity of it. We do not say that all narrations claimed to be from Imam Jafar Al-Sadiq (AS) are authentic.
The name is present in this text:
Sahl Ibn al-Hasan al-Khurasani who was one of the followers of Ahlul- Bayt in Khurasan (a province in Iran) came to Medina to meet Imam Ja’far al-Sadiq (as). Sahl al-Khurasani said: "O son of the Messenger of Allah! You are of the Imams of Ahlul-Bayt. What prevents you from getting your right (i.e., the rule) while you find more than one hundred thousand Shi’a who are ready to fight for you?”
The Imam (as) asked him to sit down, and then ordered to turn on the oven that was in the house. After the oven became very hot and turned red, the Imam (as) said: "O Khurasani step into the oven and sit in it.”The Khurasani said: "My master, O son of the Messenger of Allah! Do not punish me by fire and make it easy for me.”
At this time, Harun al-Makki (ra) entered the room, and after the greetings exchanged, the Imam (as) told him to put down his shoes and to sit down inside the oven. He did so and the Imam (as) started talking to Sahl about Khurasan as if nothing has happened.
After some time, the Imam (as) said: "O Khurasani stand up and look inside the oven.”Sahl looked into the oven and saw Harun sitting cross-legged inside the fire. Then Imam al-Sadiq (as) asked Harun to come out of oven and he came out healthy with no burn or injury.
At this time, the Imam (as) asked Sahl: "How many individuals do you know in Khurasan like this man?”
Sahl replied: "By Allah, not even one.”The Imam (as) confirmed his saying and said, "We do not rise at this time when we do not even have five helpers (like him). We know better about the proper time.”
(Bihar, v47, p123, Hadith #172; also in Manaqib by Ibn Shahr Ashub)
[I copy and pasted that from A Shi'ite Encyclopedia because I am a little lazy]
The chain of transmitters from Imam Ja'far as-Sadiq (AS) are mentioned
in books of E'lm Al-Rijaal like Rijaal Al-Kishshi and many similar books of
narrators like :
1. Wasael Al-Shia by Al-Hurr Al-Amili
2. Kitab Al-Najaashi
3. Rijal Al-Toosi
4. Al-Fihrist by Al-Toosi
5. Fihrist Muntajab al-Deen al Qummi
6. Ma'alim al-Ulama by Ibn Shahr Aashoob
7. Al-Tahreer al-Taawoosi
8. Rijal Ibn Abi Dawood
9. Jami' Al-Ruwaat by Al-Ardabili
10. Tanqeeh Al-Maqal by Al-Mamaqaani
11. Rijal Al-Kalbasi by abi Al-Huda Al-Kalbasi
12. Thiqaat Ar-Rijal by Hibat Al-Deen Al-Shahrastani
13. Mo'jam Rijal Al-Hadeeth by Al-Sayyed Al-Khoei
14. Khaatimat Kitab Wasael Al-Shia by Al-Hurr Al-Amili
15. Al-Mausoo'ah Al-Rijaaliyah Al-Muyassarah by Sheikh Al-Turabi and
Tawheed al-Mufadhdhal is very good book in this regard. You can find it in the internet.
There is no single authentic narration from Imam Jafar Al-Sadiq (AS) has ever said that his son Ismail will be the Imam after him. On the contrary, there are authentic narrations from Imam Jafar Al-Sadiq (AS) stating that Ismail is not the Imam. When Ismail passed away during the life of his father, Imam Jafar Al-Sadiq (AS) opened his coffin many times and called people to witness his death and warned them from thinking that Ismail will succeed him as an Imam.
No change in decree regarding this mater at all. Imam Musa Ibn Jafar (AS) was the Imam right from the beginning when the Prophet Muhammad (SAWA) informed about the names of the twelve Imams.
The dangerous political situation of Abbasids against Ahlul Bayt (AS), did not allow the names of some Imams to be declared in public as the tyrant rulers were after their lives. Imam Jafar Al-Sadiq (AS) made his last well for five persons to save Imam Musa Al-Kadhim (AS) from the evil of the Abbasid ruler Abu Jafar al-Mansour. Imam made his last well for : 1. His son Abdullah, 2. His wife Hameeda, 3. Al-Mansour himself, 4. The ruler of Madina on behalf of Al-Mansour and 5. Imam Musa Ibn Jafar (AS).
BY this way, Imam Jafar Al-Sadiq (AS) could save Imam Musa Al-Kadhim from the evil acts of Al-Mansour.
The claim of Imamate of Ismail was made by people supported by Abbasid rulers to weaken the unity of the Shia. As he was dead when Imam Jafar Al-Sadiq (AS was martyred, the Abbasid agents claimed his son Mohammad Ibn Ismail as an Imam.
If you read about this mater, you will clearly see that here is comparison between the knowledge and behavior of Imam Musa Al-Kadhim (AS) and Mohammad Ibn Ismail at all.
The Imamate of Imam Musa Ibn Jafar Al-Kadhim (AS) is supported by most authentic evidence, exactly like the evidence of Imamate of Imam Jafar Al-Sadiq (AS) and previous Imams (AS).
Imam Ja'far al-Sadiq (as) was a descendent of Abu Bakr through his mother. After Abu Bakr died, his wife Asma married Imam Ali, and thus Abu Bakr's son, Muhammad,was raised by 'Ali ibn Abi Talib. He became a staunch 'Alawi supporter, openly protesting against the corruption of the third caliph 'Uthman and taking part in the siege against 'Uthman's palace. He became a general in the army of Imam Ali, and fought for him at the Battle of the Camel (against Talha, Zubayr and his own sister Aisha) and at Siffeen against Mu'awiya (who had been made governor of Syria by 'Umar ibn Khattab). 'Ali ibn Abi Talib made him governor of Egypt during his short caliphate, but Ibn Abi Bakr was killed by the army of Amr ibn al-As who operated under Mu'awiya's governorship.
Our leading scholars in Elm al-Hadeeth did not confirm the authenticity of this book.
We do not say that everything in it is wrong, but cannot confirm everything mentioned in it unless and until we have authentic evidence that it was said by Imam Al-Sadiq (AS).