Musnad Ahmad 5:786

Muhammad ibn Ubayd narrated to us, Mukhtar narrated to us, from Abu Matar, who said: While we were sitting with the Commander of the Faithful, Ali, in the mosque at the Gate of al-Rahbah, a man came and said: "Show me the ablution (wudu) of the Messenger of Allah, may Allah bless him and grant him peace," and it was at the time of the sun's decline (zawal). He called Qanbar and said: "Bring me a pitcher of water." He washed his palms and his face three times, and rinsed his mouth three times, inserting some of his fingers into his mouth. He inhaled water into his nose three times, and washed his forearms three times. He wiped his head once—he said: "The inner parts of them (the ears) from the face, and the outer parts of them from the head"—and his feet up to the ankles three times, while his beard was dripping onto his chest. Then he took a sip of water after the ablution. Then he said: "Where is the one asking about the ablution of the Messenger of Allah, may Allah bless him and grant him peace? This is how the ablution of the Prophet of Allah, may Allah bless him and grant him peace, was."