Musnad Ahmad 32:1784

Aswad ibn Amir narrated to us, Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from Anas ibn Malik, that the Prophet, peace and blessings of Allah be upon him, used to pass by the house of Fatimah for six months when he went out for the dawn prayer, and he would say: "Prayer, O people of the house! {Allah only intends to remove from you the impurity, O people of the [Prophet's] household, and to purify you with [extensive] purification} [Al-Ahzab: 33]."