Musannaf Ibn Abi Shayba 3:6129

Waki' narrated to us, he said: Sufyan narrated to us, from 'Amr ibn Yahya ibn 'Umarah al-Mazini, from Sa'id ibn Yasar, from Ibn 'Umar, he said: I saw the Prophet, peace and blessings of Allah be upon him, praying upon a donkey facing the east, while he was heading toward Khaybar.