Sunan Abi Dawud Book 4, Hadith 30
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.
I saw the Messenger of Allah (ﷺ) praying on a donkey while he was facing Khaibar.
Ata' ibn AbuRabah asked Aisha: Can women offer prayer on a riding beast? She replied: They were not permitted to do so in hardship or comfort. Muhammad ibn Shu'ayb said: This (prohibition) applies to the obligatory prayers.