Maliks Muwatta Book 9, Hadith Number 24.

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

Yahya related to me from Malik from Nafi that Abdullah ibn Umar never used to pray anything with the fard prayer, either before it or after it, while travelling, except in the depths of the night. He would pray on the ground or on his mount, whichever way it was facing.