Musnad Ahmad 5:450

Abdullah narrated to us, Ishaq ibn Isma'il narrated to me, Sufyan narrated to us, from Abu al-Sawda', from Ibn 'Abd Khayr, from his father, who said: I saw Ali perform ablution, and he washed the tops of his feet. He said: "Had I not seen the Messenger of Allah, peace and blessings of Allah be upon him, washing the tops of his feet, I would have thought that the bottoms of them were more deserving of being washed."