Musannaf Ibn Abi Shayba 1:183

Waki’ narrated to us, from al-A’mash, from Abu Ishaq, from ‘Abd Khayr, from ‘Ali, who said: If the religion were based on opinion, the bottom of the feet would be more deserving of being wiped than the top of them. However, I saw the Messenger of Allah (ﷺ) wipe the top of them.