Musannaf Ibn Abi Shayba 28:1177

Waki’ narrated to us, he said: ‘Umar ibn Husayl narrated to us, he said: I heard al-Sha’bi saying: Dihya al-Kalbi said: “O Messenger of Allah, shall we not mount a donkey upon a mare so that it produces a mule that we may ride?” He said: “Only those who do not know do that.”