Musnad Ahmad 1060:27

Muhammad ibn Ja'far narrated to us, he said: Ma'mar ibn Rashid dictated to me, he said: Al-Zuhri informed us, from 'Ata' ibn Yazid, from Abu Ayyub al-Ansari, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When one of you goes to relieve himself, let him not face the Qiblah, but let him face the east or the west." He said: "When we arrived in the Levant, we found toilets that had been built facing the Qiblah, so we would turn away and ask Allah for forgiveness."