Musnad Ahmad 5:204

Yahya ibn Adam narrated to us, Sharik narrated to us, from Uthman ibn Abi Zur'ah, from Salim ibn Abi al-Ja'd, from Ali ibn Alqamah, from Ali, who said: A mule or a female mule was gifted to the Messenger of Allah, may Allah bless him and grant him peace. I said: "What is this?" He said: "A mule, or a female mule." I said: "And from what thing is it?" He said: "The donkey is mounted upon the mare, and this comes out between them." I said: "Shall we not mount such-and-such [male] upon such-and-such [female]?" He said: "No, only those who do not know do that."