Musnad Ahmad 27:1002

'Affan narrated to us, Hammad ibn Salamah narrated to us, 'Amr ibn Yahya narrated to us, from Sa'id ibn Yasar, from Ibn 'Umar, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) praying upon a donkey or a female donkey while he was heading toward Khaybar.