Musannaf Ibn Abi Shayba 3:6134

Ibn ‘Ulayyah narrated to us, from Hisham al-Dastawa’i, from Yahya ibn Abi Kathir, from Muhammad ibn ‘Abd al-Rahman, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to pray upon his riding beast facing the east. When he wanted to pray the obligatory prayer, he would descend and face the Qibla.