Maliks Muwatta Book 9, Hadith Number 29.

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

Yahya related to me from Malik that Yahya ibn Said said, “I saw Anas ibn Malik on a journey praying on a donkey facing away from the qibla. He did the raka and the sajda by motioning with his head, without putting his face on anything.”