Musnad Ahmad 554:1

Abu al-Nadr narrated to us, Dawud—meaning al-Attar—narrated to us, from Amr ibn Yahya, from Abu Zayd, the freed slave of Tha'labah, from Ma'qil ibn Abi Ma'qil al-Asadi, that the Messenger of Allah (peace and blessings of Allah be upon him) forbade us from facing either of the two Qiblahs while urinating or defecating.