Musnad Ahmad 26:322

Yunus narrated to us, Layth narrated to us, from Yazid ibn Abi Habib, from Muhammad ibn Ishaq, from Abd al-Rahman ibn al-Aswad, who narrated to him, from his father, that Ibn Mas’ud narrated to him, that the Messenger of Allah (peace and blessings of Allah be upon him) "would generally turn away from the prayer to his left toward the chambers."