Abu Dawud Book 1, Hadith Number 164.
Chapter : How to wipe over the socks.
Narated By N/A : A’mash 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 (PBUH) 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 (PBUH) doing like this – and he narrated the tradition in full.