Sahih Muslim 701
Sahih Muslim · Book 6: The Book of Prayer - Travellers · It is permissible to offer voluntary prayers atop one’s mount when travelling, no matter what direction it is facing
وَحَدَّثَنَا عَمْرُو بْنُ سَوَّادٍ ، وَحَرْمَلَةُ ، قَالاَ أَخْبَرَنَا ابْنُ وَهْبٍ ، أَخْبَرَنِي يُونُسُ ، عَنِ ابْنِ شِهَابٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عَامِرِ بْنِ رَبِيعَةَ ، أَخْبَرَهُ أَنَّ أَبَاهُ أَخْبَرَهُ أَنَّهُ، رَأَى رَسُولَ اللَّهِ صلى الله عليه وسلم يُصَلِّي السُّبْحَةَ بِاللَّيْلِ فِي السَّفَرِ عَلَى ظَهْرِ رَاحِلَتِهِ حَيْثُ تَوَجَّهَتْ .
'Abdullah b. 'Amir b. Rabi'a has reported on the authority of his father that he had seen the Messenger of Allah (ﷺ) observing Nafl player at night on a journey on the back of his ride in whichever direction it turned its face.