Musnad Ahmad 478:4

Abu Amir narrated to us, Ali—meaning Ibn al-Mubarak—narrated to us, from Yahya, from Abu Salamah, who said: Ja'far ibn 'Amr ibn Umayyah informed me, from his father, that he saw the Prophet, peace and blessings of Allah be upon him, wiping over the khuffayn (leather socks).