I saw the Prophet ﷺ on his Mount praying Nawafil by nodding his head, whatever direction he faced, but Messenger of Allah ﷺ never did the same in offering the compulsory prayers.  

أَخْبَرَهُ قَالَ رَأَيْتُ رَسُولَ اللَّهِ ﷺ وَهْوَ عَلَى الرَّاحِلَةِ يُسَبِّحُ يُومِئُ بِرَأْسِهِ قِبَلَ أَىِّ وَجْهٍ تَوَجَّهَ وَلَمْ يَكُنْ رَسُولُ اللَّهِ ﷺ يَصْنَعُ ذَلِكَ فِي الصَّلاَةِ الْمَكْتُوبَةِ  

At night ʿAbdullah bin ʿUmar used to offer the prayer on the back of his animal during the journey and never cared about the direction he faced. Ibn ʿUmar said, "Messenger of Allah ﷺ used to offer the optional prayer on the back of his Mount facing any direction and also used to pray the witr on it but never offered the compulsory prayer on it."  

سَالِمٌ كَانَ عَبْدُ اللَّهِ يُصَلِّي عَلَى دَابَّتِهِ مِنَ اللَّيْلِ وَهْوَ مُسَافِرٌ مَا يُبَالِي حَيْثُ مَا كَانَ وَجْهُهُ قَالَ ابْنُ عُمَرَ وَكَانَ رَسُولُ اللَّهِ ﷺ يُسَبِّحُ عَلَى الرَّاحِلَةِ قِبَلَ أَىِّ وَجْهٍ تَوَجَّهَ وَيُوتِرُ عَلَيْهَا غَيْرَ أَنَّهُ لاَ يُصَلِّي عَلَيْهَا الْمَكْتُوبَةَ  

The Prophet ﷺ used to pray (the Nawafil) on his Mount facing east and whenever he wanted to offer the compulsory prayer, he used to dismount and face the Qibla.  

أَنَّ النَّبِيَّ ﷺ كَانَ يُصَلِّي عَلَى رَاحِلَتِهِ نَحْوَ الْمَشْرِقِ فَإِذَا أَرَادَ أَنْ يُصَلِّيَ الْمَكْتُوبَةَ نَزَلَ فَاسْتَقْبَلَ الْقِبْلَةَ