Musannaf Ibn Abi Shayba 1:1872

Zayd ibn al-Hubab narrated to us, from Khalid ibn Abi Bakr, he said: Salim ibn Abdullah informed me, from his father, that Sa’d ibn Abi Waqqas asked Umar ibn al-Khattab about wiping over the leather socks (khuffayn). Umar said: I heard the Prophet, may Allah bless him and grant him peace, commanding the wiping over the leather socks when one puts them on while they are in a state of purity.