Musannaf Ibn Abi Shayba 1:191

Humaid ibn Abd al-Rahman narrated to us, from al-Hasan ibn Salih, from Abu al-Hijaf, from al-Hakam, who said: I heard him saying: The Sunnah from the Messenger of Allah, peace and blessings of Allah be upon him, and the Muslims has passed—meaning: regarding the washing of the two feet.