It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) took a garment and placed it over Sayyiduna Ali, Sayyidah Fatimah, Sayyiduna Hasan and Sayyiduna Husayn (may Allah be pleased with them), then recited this verse: "Allah only intends to remove impurity from you, O people of the household, and to purify you thoroughly" [Surah al-Ahzab: 33].
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20086