Sunan Abi Dawud Book 4, Hadith 29
I saw the Messenger of Allah (ﷺ) praying on a donkey while he was facing Khaibar.
When the Messenger of Allah (ﷺ) was on a journey and wished to say voluntary prayer, he made his she-camel face the qiblah and uttered the takbir (Allah is most great), then prayed in whatever direction his mount made his face.
The Messenger of Allah (ﷺ) sent me on some business, and when I came to him he was praying on (the back of) his riding beast (moving) towards the east and making the prostration lower than the bowing.