Musannaf Ibn Abi Shayba 16:948

Abu Bakr narrated to us, he said: Waki' narrated to us, he said: Sawadah bin Abi al-Aswad narrated to us, from his father, from Abdullah bin Amr, who said: "Stick to these black leather socks (khuffayn) and wear them, for they are more worthy for you to wipe over."