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).

— Sahih Bukhari, Volume 1, Book 9: Virtues of the Prayer Hall (Sutra of the Musalla), Hadith #485