Musnad Ahmad 447:11

Muhammad ibn Mus'ab narrated to us, he said: Al-Awza'i narrated to us, from Shaddad Abu 'Ammar, he said: I entered upon Wathilah ibn al-Asqa', and there were people with him, and they mentioned 'Ali. When they stood up, he said to me: "Shall I not inform you of what I saw from the Messenger of Allah, may Allah bless him and grant him peace?" I said: "Yes." He said: "I came to Fatimah, may Allah be pleased with her, asking her about 'Ali. She said: 'He has gone to the Messenger of Allah, may Allah bless him and grant him peace.' So I sat waiting for him until the Messenger of Allah, may Allah bless him and grant him peace, came, and with him were 'Ali, Hasan, and Husayn, may Allah be pleased with them, holding the hand of each one of them, until he entered. He brought 'Ali and Fatimah close and seated them in front of him, and he seated Hasan and Husayn, each one of them on his thigh. Then he wrapped his garment—or he said: his cloak—around them, then he recited this verse: 'Allah only intends to remove [all] impurity from you, O people of the [Prophet's] household, and to purify you with [extensive] purification' [Al-Ahzab: 33]. And he said: 'O Allah, these are the people of my household, and the people of my household are more deserving.'"