The Ahlul Bayt is the household of the Prophet Muhammed S.A.W. Household here means the nearest relatives referring to his daughter Fatima al-Zahra, his son-in-law and successor Imam Ali A.S., their two son Imam Hasan and Imam Husayn and the nine Imams from the lineage of Imam Husayn.
From the holy Quran in Chapter 33 (Al-Ahzab), verse 33 Allah say;
“…Allah only desires to keep away the uncleanness from you, O people of the House! and to purify you a (thorough) purifying….”
The Messenger of Allah (S) in his well-attested tradition said: “Verily, I leave behind amongst you two precious things: the Book of Allah, the Almighty and the Glorious, and my kindred (`Itrah). The two will never be separate until they return to me by the Pond.”