Sahih Bukhari
Virtues of the Prayer Hall (Sutra of the Musalla)
Volume: 001 / Book Number: 009 / Hadith Number: 485
Narrated Nafi:
"The Prophet used to make his she-camel sit across and he would pray facing it (as a Sutra)." I asked, "What would the Prophet do if the she-camel was provoked and moved?" He said, "He would take its camel-saddle and put it in front of him and pray facing its back part (as a Sutra). And Ibn 'Umar used to do the same." (This indicates that one should not pray except behind a Sutra).