Musnad Ahmad 5:697

Abu Nu'aym narrated to us, Yunus narrated to us, from Abu Ishaq, from 'Abd Khayr, who said: I saw 'Ali perform ablution and wipe over his sandals. Then he said: "Were it not that I saw the Messenger of Allah (may Allah bless him and grant him peace) do as you saw me do, I would have thought that the bottom of the feet is more deserving of being wiped than the top of them."