Musnad Ahmad 1163:32

Abu Bakr al-Hanafi narrated to us, he said: Umar ibn Ishaq ibn Yasar narrated to us, he said: I read in a book belonging to Ata ibn Yasar, with Ata ibn Yasar, he said: So I asked Maymunah, the wife of the Prophet, may God bless him and grant him peace, about wiping over the leather socks (khuffayn). She said: I said, "O Messenger of God, does a person wipe over the leather socks every hour without taking them off?" He said, "Yes."