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. (Using translation from Bukhārī 1099)  

«أَنَّ رَسُولَ اللَّهِ ﷺ كَانَ يُصَلِّي عَلَى رَاحِلَتِهِ نَحْوَ الْمَشْرِقِ فَإِذَا أَرَادَ أَنْ يُصَلِّيَ الْمَكْتُوبَةَ نَزَلَ فَاسْتَقْبَلَ الْقِبْلَةَ»  

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. (Using translation from Bukhārī 1097)  

«رَأَيْتُ رَسُولَ اللَّهِ ﷺ يُسَبِّحُ وَهُوَ عَلَى الرَّاحِلَةِ وَيُومِئُ بِرَأْسِهِ قِبَلَ أَيِّ وَجْهٍ تَوَجَّهَ وَلَمْ يَكُنْ رَسُولُ اللَّهِ ﷺ يَصْنَعُ ذَلِكَ فِي الصَّلَاةِ الْمَكْتُوبَةِ»