Sahih Bukhari Volume 001, Book 009, Hadith Number 485.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 09. Virtues Of The Prayer Hall (Sutra Of The Musalla)
Narated By 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).