Musnad Abu Ya'la al-Mawsili 110:18

Abu Bakr ibn Abi Shaybah narrated to us, Abu al-Ahwas narrated to us, from Simak, from Ikrimah, from Ibn Abbas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not face the Qiblah (while relieving yourselves), do not turn your backs to it, and do not sell to one another (in a way that involves deception or hoarding)."