Bukhari 507

From Sahih Bukhari (Hadith 507):

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)