"Ahl al-Bayt" has been mentioned in the Qur'an in three places:
Verse 73 of Surah Hud, where it refers to prophet Abraham (a) and his wife, "They said, 'Are you amazed at Allah's dispensation? [That is] Allah's mercy and His blessings upon you, members of the household. Indeed He is all-laudable, all-glorious.'" (11:73)
Verse 12 of Surah Qasas which refers to the people of prophet Moses (s), "We had forbidden him to be suckled by any nurse since before. So she1 said, 'Shall I show you a household that will take care of him for you and will be his well-wishers?'" (28:12)
Verse 33 of Surah Ahzab which is known as the verse of Tathir, where God addresses the Prophet (s) and his family, "Indeed Allah desires to repel all impurity from you, O People of the Household, and purify you with a thorough purification." (33:33)
About the question over who is meant in this verse by the "Ahl al-Bayt", there are different opinions. An opinion agreed by Shi'a and many Sunni scholars is that it refers to the Ahl al-Kasa', i.e. the Prophet (s), 'Ali (a), Fatima (s), al-Hasan (a) and al-Husayn (a).