Maliks Muwatta Book 9, Hadith Number 28.

Section : Voluntary Prayers while Travelling, by Day and at Night, and Praying on a Riding Beast.

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.”