Musannaf Ibn Abi Shayba 1:1930

Abu Bakr narrated to us, he said: Yahya ibn Ya'la narrated to us, from Layth, from al-Hakam, from Ibn Abi Layla, from Ka'b, from Bilal: That the Prophet, peace and blessings of Allah be upon him, and Abu Bakr and Umar used to wipe over the leather socks (khuffayn) and the head covering (khimar).