Narrated Nafi:

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