What is the ruling on crossbreeding a female donkey with a horse to produce a mule?

The summary of the answer: It was narrated from Ibn Abbas, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, forbade mating donkeys with horses. However, scholars understood this prohibition as an encouragement to increase the number of horses. Therefore, there is no objection to crossbreeding between horses and donkeys.