Abu Dawud Book 2, Hadith Number 1224.

Chapter : To offer the obligatory prayer on a riding beast on account of some excuse.

Narated By ‘Aisha, Ummul Mu’minin : 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.