Musnad Ahmad 25:140

Isma'il narrated to us, Musa ibn Salim Abu Jahdam narrated to us, 'Abdullah ibn 'Ubaydullah ibn 'Abbas narrated to me, he heard Ibn 'Abbas say: "The Messenger of Allah (peace and blessings of Allah be upon him) was a servant who was commanded. By Allah, he conveyed what he was sent with, and he did not single us out over the people with anything except for three things. He commanded us to perform ablution thoroughly, not to consume charity, and not to mount a donkey upon a mare." Musa said: I met 'Abdullah ibn Hasan and said: "'Abdullah ibn 'Ubaydullah narrated to me such and such." He said: "Indeed, horses were few among the Banu Hashim, so he liked for them to increase among them."