Language:
كتاب الصلاة
8  
Prayers (Salat)
(98)
Chapter: To offer As-Salat (the prayer) facing a Rahila camel, a tree or a camel saddle as a Sutra).
(98)
باب الصَّلاَةِ إِلَى الرَّاحِلَةِ وَالْبَعِيرِ وَالشَّجَرِ وَالرَّحْلِ
Sahih al-Bukhari 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).

حَدَّثَنَا مُحَمَّدُ بْنُ أَبِي بَكْرٍ الْمُقَدَّمِيُّ، حَدَّثَنَا مُعْتَمِرٌ، عَنْ عُبَيْدِ اللَّهِ، عَنْ نَافِعٍ، عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ كَانَ يُعَرِّضُ رَاحِلَتَهُ فَيُصَلِّي إِلَيْهَا‏.‏ قُلْتُ أَفَرَأَيْتَ إِذَا هَبَّتِ الرِّكَابُ‏.‏ قَالَ كَانَ يَأْخُذُ هَذَا الرَّحْلَ فَيُعَدِّلُهُ فَيُصَلِّي إِلَى آخِرَتِهِ ـ أَوْ قَالَ مُؤَخَّرِهِ ـ وَكَانَ ابْنُ عُمَرَ ـ رضى الله عنه ـ يَفْعَلُهُ‏.‏
Reference : Sahih al-Bukhari 507
In-book reference : Book 8, Hadith 154
USC-MSA web (English) reference : Vol. 1, Book 9, Hadith 485
  (deprecated numbering scheme)