Musnad Ahmad 5:175

Waki’ narrated to us, al-A’mash narrated to us, from Abu Ishaq, from ‘Abd Khayr, from ‘Ali, who said: I used to think that the bottom of the feet were more deserving of being wiped than their tops, until I saw the Messenger of Allah (peace and blessings of Allah be upon him) wiping their tops.