Yahya related to me from Malik from Yahya ibn Said from Muhammad ibn Yahya ibn Habban from his paternal uncle, Wasi ibn Habban, that Abdullah ibn Umar said, "People say, 'When you sit to relieve yourself, do not face the qibla or the Bayt al-Maqdis.' " Abdullah continued, "I went upon top of a house of ours and saw the Messenger of Allah, may Allah bless him and grant him peace, (squatting) on two unfired bricks facing the Bayt al-Maqdis, relieving himself." Ibn Umar added, "Perhaps you are one of those who pray folded on their haunches." Wasi replied, "I don't know, by Allah!"
USC-MSA web (English) reference: Book 14, Hadith 3
Arabic reference: Book 14, Hadith 460