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.‏