Musannaf Ibn Abi Shayba 1:1601

Shababa narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, from 'Ata' ibn Yazid, from Abu Ayyub, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you goes for the call of nature, do not face the Qibla, and do not turn your back towards it. Face the east or the west."