Sunnah.com reference : Book 2, Hadith 78English translation : Book 2, Hadith 212Arabic reference : Book 2, Hadith 214
Narrated Anas in another narration of the Hadith reported by Abu Dawud:
When [he (ﷺ)] traveled, and intended to pray a voluntary prayer, he used to direct his riding camel towards the Qiblah, say, "Allahu Akbar" and pray facing whatever direction it faced.

[Its chain is Hasan (good)].
وَلِأَبِي دَاوُدَ : مِنْ حَدِيثِ أَنَسٍ : { كَانَ إِذَا سَافَرَ فَأَرَادَ أَنْ يَتَطَوَّعَ اِسْتَقْبَلَ بِنَاقَتِهِ اَلْقِبْلَةِ , فَكَبَّرَ , ثُمَّ صَلَّى حَيْثُ كَانَ وَجْهَ رِكَابِهِ }  وَإِسْنَادُهُ حَسَنٌ .‏  1‏‏1 ‏- حسن .‏ رواه أبو داود (1225)‏ وصححه غير واحد .‏