This section of Al-Islam.org contains information about the Most Noble Messenger, Muhammad, his household (Ahlul bayt), and their companions, peace be upon them all.
The Ahlul Bayt or household of the Prophet Muhammad - peace be upon him and his household [pbuh] - refers to his daughter Fatima al-Zahra, his successor and son-in-law, Imam Ali, their two sons Imam Hasan and Imam Husayn, and the nine Imams from the lineage of Imam Husayn, peace be upon them all.
The Holy Qur'an
Regarding them, Allah (God), the most Glorified and High, says in the Holy Qur'an:
Muhammad [pbuh], the Prophet of Islam
The Prophet of Islam [peace upon him and his household], in the well-attested tradition called Hadith al-Thaqalayn said:
Verily, I am leaving behind two precious things (thaqalayn) among you: the Book of God and my kindred (`itrah), my household (Ahl alBayt), for indeed, the two will never separate until they come back to me by the Pond (of alKawthar on the Judgement's Day).[Source: Please see Hadith al-Thaqalayn: A Study of its Tawatur]
Ali [a], the first Imam, successor to the Prophet
"...We the people of the house (of the Prophet - Ahlul bayt) possess the doors of wisdom and light of governance. Beware that the paths of religion are one and its highways are straight. He who follows them achieves (the aim) and secures (the objective). And he who stood away from it went astray and incurred repentance." [Sermon 119]
"We are the near ones, companions, treasure holders and doors (to the Sunnah). Houses are not entered save through their doors. Whoever enters them from other than the door is called a thief." [Sermon 153]
"Where are those who falsely and unjustly claimed that they are deeply versed in knowledge, as against us, although Allah raised us in position and kept them down, bestowed upon us knowledge but deprived them, and entered us (in the fortress of knowledge) but kept them out. With us guidance is to be sought and blindness (of misguidance) is to be changed into brightness. Surely Imams (divine leaders) will be from the Quraysh. They have been planted in this line through Hashim. It would not suit others nor would others be suitable as heads of affairs." [Sermon 143]
[Source: Nahjul Balagha, translated by Syed Ali Raza]
"We are lights of the heavens and the earth and the ships of salvation. We are the repository of knowledge, and towards us is the home-coming of all the matters. Through our Mahdi all (false) arguments shall be refuted, and he is the seal of the Imams, the deliverer of the Ummah, and the extremity of the light. Happy are those who hold on to our handle and are brought together upon our love.
[Source: Sibt Ibn al-Jawzi, Tadhkirat al-khawass, p. 138]
Therefore, based on the above and other lines of evidence, it is the belief of Islam that the Holy Prophet and his family [peace be upon them] were blessed by Allah and purified to the level of being pure and sinless throughout their distinguished lives.