Musannaf Ibn Abi Shayba 1:1600

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, from Ibrahim, from 'Abd al-Rahman ibn Yazid, he said: They said to Salman: "Your Prophet has taught you everything, even how to relieve yourselves." He said: "Yes, he has forbidden us from facing the Qiblah while defecating or urinating."