Musnad Abu Ya'la al-Mawsili 111:24

Abu Ya'la informed us, he said: It was read to Bishr ibn al-Walid, Abu Yusuf informed you, from 'Amr ibn Yahya al-Ansari, from Sa'id, the freed slave of Shaqran, the freed slave of the Messenger of Allah (may Allah bless him and grant him peace), that Ibn 'Umar narrated to him: That the Messenger of Allah (may Allah bless him and grant him peace) used to pray upon a donkey while he was heading toward Khaybar.