Mishkat al-Masabih 4 Prayer كتاب الصلاة 752

وَعَنْ أَنَسٍ قَالَ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ إِذَا سَافَرَ وَأَرَادَ أَنْ يَتَطَوَّعَ اسْتَقْبَلَ الْقِبْلَةَ بِنَاقَتِهِ فَكَبَّرَ ثُمَّ صَلَّى حَيْثُ وَجهه ركابه. رَوَاهُ أَبُو دَاوُد

Anas said that when God’s Messenger was on a journey and wished to say voluntary prayers he made his she-camel face the qibla and said, “God is most great,” then prayed in whatever direction his mount made him face. Abu Dawud transmitted it.