Musnad Ahmad 5:64

Isma'il narrated to us, Muhammad ibn Ishaq narrated to us, Muhammad ibn Talhah ibn Yazid ibn Rukanah narrated to me, from Ubaydullah al-Khawlani, from Ibn Abbas, who said: Ali entered upon me in my house, and he called for water for ablution. We brought him a bowl that held a mudd or close to it, and it was placed before him, and he had urinated. He said: "O Ibn Abbas, shall I not perform for you the ablution of the Messenger of Allah, may Allah bless him and grant him peace?" I said: "Yes, may my father and mother be sacrificed for you." He said: "So a vessel was placed for him, and he washed his hands, then he rinsed his mouth, inhaled water, and blew it out. Then he took with his hands and struck his face with them, and placed his thumbs against the parts of his ears that were facing forward." He said: "Then he repeated that three times. Then he took a handful of water with his right hand and poured it onto his forehead, then let it flow down his face. Then he washed his right hand up to the elbow three times, then his other hand likewise. Then he wiped his head and his ears from their backs. Then he took water with his palms and struck his feet with them while they were in his sandals, then he turned them with them, then [he did] the same to the other foot." I said: "Even while in the sandals?" He said: "Even while in the sandals." I said: "Even while in the sandals?" He said: "Even while in the sandals." I said: "Even while in the sandals?" He said: "Even while in the sandals."