Sunan Abu Dawood, Chapter: Purification (Kitab Al-Taharah)

Amash transmitted this tradition saying: If the religion were based on opinion, it would be more proper to wipe the under part of the feet than the upper. The Prophet (peace_be_upon_him) wiped over the upper part of his shoes. The narrator Waki' said: By feet he meant socks. Another version says: I saw 'Ali perform ablution and wash the upper part of his feet, and say: Had I not seen the Messenger of Allah (peace_be_upon_him) doing like this - and he narrated the tradition in full.