Musnad Ahmad 626:24

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, he said: I heard Bakr ibn ‘Abdullah narrating from al-Mughirah ibn Shu‘bah, that he said: "Two matters I do not ask anyone among the people about, for I saw the Messenger of Allah, peace and blessings of Allah be upon him, do them: The prayer of the Imam behind a man from his subjects, and I have seen the Messenger of Allah, peace and blessings of Allah be upon him, pray behind ‘Abd al-Rahman ibn ‘Awf one rak‘ah of the morning prayer. And a man wiping over his leather socks, and I have seen the Messenger of Allah, peace and blessings of Allah be upon him, wiping over the leather socks."