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).