بُلُوغُ الْمَرَامِ
Bulugh al-Maram 214
The Book of Prayer
وَلِأَبِي دَاوُدَ : مِنْ حَدِيثِ أَنَسٍ : { كَانَ إِذَا سَافَرَ فَأَرَادَ أَنْ يَتَطَوَّعَ اِسْتَقْبَلَ بِنَاقَتِهِ اَلْقِبْلَةِ , فَكَبَّرَ , ثُمَّ صَلَّى حَيْثُ كَانَ وَجْهَ رِكَابِهِ } وَإِسْنَادُهُ حَسَنٌ . [1]
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)].
Sunnah.com reference: Book 2, Hadith 78