I saw the Messenger of Allah ﷺ praying voluntary prayers on the back of his mount, facing whatever direction it turned.
حدثنا عبد الرزاق قال أخبرنا معمر عن الزهري
رأيت رسول الله ﷺ يصلي على ظهر راحلته النوافل في كل جهة
Its isnad is sahih, it was mentioned previously at 15612.