(98) باب الصَّلاَةِ إِلَى الرَّاحِلَةِ وَالْبَعِيرِ وَالشَّجَرِ وَالرَّحْلِ
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).
Reference: Sahih al-Bukhari 507
In-book reference: Book 8, Hadith 154
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 9, Hadith 485