Musannaf Ibn Abi Shayba 1:219

Abu Mu'awiyah narrated to us, from al-A'mash, from al-Hakam, from 'Abd al-Rahman ibn Abi Layla, from Ka'b ibn 'Ujrah, from Bilal, that the Messenger of Allah (may Allah bless him and grant him peace) wiped over the two leather socks (khuffayn) and the head covering (khimar).