Musannaf Ibn Abi Shayba 1:1603

Khalid ibn Makhlad narrated to us, from Sulayman ibn Bilal, he said: Amr ibn Yahya al-Mazini narrated to me, from Abu Zayd, from Ma'qil al-Asadi—who had accompanied the Prophet (may Allah bless him and grant him peace)—he said: The Messenger of Allah (may Allah bless him and grant him peace) forbade facing either of the two qiblas while defecating or urinating.