Musnad Ahmad 1060:17

Ishaq ibn Isa narrated to us, Malik informed us, from Ishaq ibn Abdullah, from Rafi ibn Ishaq, the freed slave of Abu Talha, that he heard Abu Ayyub al-Ansari saying while he was in Egypt: "By Allah, I do not know what to do with these latrines," meaning the toilets, "for the Messenger of Allah (ﷺ) said: 'When one of you goes for defecation or urination, do not face the Qibla nor turn your back toward it.'"