Muwatta Malik Book 9, Hadith 30
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."
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 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."