Musnad Ahmad 26:833

Hajjaj narrated to us, Layth ibn Sa'd narrated to us, Yazid ibn Abi Habib narrated to me, from Muhammad ibn Ishaq, that Abd al-Rahman ibn al-Aswad narrated to him, that al-Aswad narrated to him, that Ibn Mas'ud narrated to him, that the Messenger of Allah (peace and blessings of Allah be upon him): "The majority of the time he would turn away from the prayer toward his left, toward the chambers."