Musnad Abu Ya'la al-Mawsili 2:31

Abu Kurayb Muhammad ibn al-Ala narrated to us, Zayd ibn al-Hubab narrated to us, Khalid ibn Abi Bakr ibn Ubayd Allah al-Umari narrated to us, he said: Salim narrated to me, from his father, that Sa’d ibn Abi Waqqas asked Umar about wiping [over the khuffayn]. Umar said: I heard the Messenger of Allah, may Allah bless him and grant him peace, commanding to wipe over the top of the khuffayn if he puts them on while they are pure.