Musnad Abu Ya'la al-Mawsili 3:86

Abu Khaythama narrated to us, Waki’ ibn al-Jarrah 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 be upon him) wiping their tops."