Muwatta Malik Book 9, Hadith 29
Yahya related to me from Malik from Amr ibn Yahya al-Mazini from
Abu'l-Hubab Said ibn Yasar that Abdullah ibn Umar said, "I saw the
Messenger of Allah, may Allah bless him and grant him peace, praying
on a donkey while heading towards Khaybar."
Yahya related to me that Malik said, "I have heard from Nafi that Abdullah ibn Umar used to see his son, Ubaydullah ibn Abdullah, doing voluntary prayers on a journey,and he would not disapprove of it."
Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, used to pray on his mount while travelling, whichever way it was facing. Abdullah ibn Dinar said, "Abdullah ibn Umar would also do that."