Final answer:
The largest army that the Muslims had fought before the Battle of Mutah was the Byzantine army at the Battle of Yarmuk in 636.
Step-by-step explanation:
The largest army that the Muslims had fought before the Battle of Mutah was the Byzantine army at the Battle of Yarmuk in 636. The Arab-Muslim armies achieved a significant victory against the Byzantines and captured the city of Damascus. This success led to the Byzantines abandoning Greater Syria, which included present-day Israel, Jordan, Lebanon, Palestine, and Syria.