Maliks Muwatta Book 9, Hadith Number 44.
Section : The Sutra of a Man Praying on a Journey.
Yahya related to me from Malik that he had heard that Abdullah ibn Umar would use the animal he was riding as a sutra when he prayed.
Yahya related to me from Malik from Hisham ibn Urwa that his father used to pray in the desert without a sutra.