Sahih Bukhari

Narrated Nafi`:""The Prophet (ﷺ) used to make his she-camel sit across and he would pray facing it (as a Sutra)."" I asked, ""What would the Prophet (ﷺ) 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)