Umar returned from Syria because Muhammad once instructed that one should not enter the place where an epidemic is unless it is absolutely safe. So Abu Ubaidah returned to his army at Emesa. It was then that a plague hit the land of Syria, the like of which people had never experienced before. It devastated the population. As Caliph Umar wanted to make Abu Ubaidah his successor he didn't want him to remain there in the epidemic region. Umar dispatched a messenger to Abu Ubaidah with a letter saying: