I saw the Messenger of Allah (ﷺ) praying (Nafl prayer) on a donkey's back while his face was turned towards Khaibar.
Sahih Muslim Book 6, Hadith 45
← Previous Hadith " Whether you turn thither is Allah's face," and it was revealed in this context.

Sahih Muslim Book 6, Hadith 44 Next Hadith → I was travelling along with Ibn 'Umar on the way to Mecca. Sa'id said: When I apprehended dawn, I dismounted (the ride) and observed Witr prayer and then again joined him. Ibn 'Umar said to me: Where were you? I said: I apprehended the appearance of dawn, so I dismounted and observed Witr prayer. Upon this 'Abdullah said: Is there not a model pattern for you in the Messenger of Allah (ﷺ)? I said: Yes, by Allah, and (then) he said: The Messenger of Allah (ﷺ) used to observe Witr prayer on the camel's back.

Sahih Muslim Book 6, Hadith 46