Musnad Ahmad 5:355

'Abdullah narrated to us, Ishaq bin Isma'il narrated to us, 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) wash the tops of his feet, I would have thought that the bottoms of them were more deserving of being washed."