It was narrated that Ma'qil bin Abu Ma'qil Al-Asadi, who was a Companion of the Prophet, said:
"The Messenger of Allah forbade us from facing either of the two Qiblah when defecating or urinating." (JA)
English reference (JA): Vol. 1, Book 1, Hadith 319 (JA)
Arabic reference (JA): Book 1, Hadith 340 (JA)