Sunan Abi Dawud Book 4, Hadith 31
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.
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 went on an expedition with the Messenger of Allah (ﷺ), and I was present with him at the conquest. He stayed eighteen days in Mecca and prayed only two rak'ahs (at each time of prayer). And he said: You who live in the town must pray four; we are travellers.