مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2090
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: The Messenger of Allah (ﷺ) forbade mating a donkey with a mare.
Musnad Ahmad 2092Its isnad is saheeh]