Musnad Ahmad 5:223

Hashim narrated to us, Layth—meaning Ibn Sa’d—narrated to us, from Yazid ibn Abi Habib, from Abu al-Khayr, from ‘Abdullah ibn Zurayr al-Ghafiqi, from ‘Ali ibn Abi Talib, that he said: A mule was gifted to the Messenger of Allah, may Allah bless him and grant him peace. We said: "O Messenger of Allah, what if we were to mate donkeys with our horses, so that they might produce for us the likes of this?" The Messenger of Allah, may Allah bless him and grant him peace, said: "Only those who do not know do that."