Musnad Ahmad 26:832

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: Abd al-Rahman ibn al-Aswad ibn Yazid al-Nakha'i narrated to me, from his father, who said: I heard a man asking Abdullah ibn Mas'ud about the turning away of the Messenger of Allah, may Allah bless him and grant him peace, from his prayer: did he turn away to his right or to his left? He said: Abdullah ibn Mas'ud said: The Messenger of Allah, may Allah bless him and grant him peace, would turn away however he wished. The most frequent turning away of the Messenger of Allah, may Allah bless him and grant him peace, from his prayer was on his left side toward his chamber.