Musnad Abu Ya'la al-Mawsili 133:1

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Hammad ibn Salamah narrated to us, Ali ibn Zayd narrated to us, from Anas, that the Prophet, peace and blessings of Allah be upon him, would pass by the door of Fatimah, the daughter of the Prophet, for six months at the time of the dawn prayer. He would say, "The prayer, O people of the house," three times. "Allah only intends to remove from you the impurity, O people of the house, and to purify you with [extensive] purification" (Al-Ahzab).