"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 ʿUmar used to do the same." (This indicates that one should not pray except behind a Sutra).  

عَنِ النَّبِيِّ ﷺ أَنَّهُ كَانَ يُعَرِّضُ رَاحِلَتَهُ فَيُصَلِّي إِلَيْهَا قُلْتُ أَفَرَأَيْتَ إِذَا هَبَّتِ الرِّكَابُ قَالَ كَانَ يَأْخُذُ هَذَا الرَّحْلَ فَيُعَدِّلُهُ فَيُصَلِّي إِلَى آخِرَتِهِ أَوْ قَالَ مُؤَخَّرِهِ وَكَانَ ابْنُ عُمَرَ ؓ يَفْعَلُهُ