Byzantines
According to traditional Islamic sources, in 628 Muhammad sent a letter to Heraclius inviting him to Islam. The Byzantine emperor received it while on a pilgrimage in Jerusalem and called upon a person who belonged to Muhammad's tribe Quraysh. According to Sunni tradition, Abu Sufyan ibn Harb came forward and a discussion between them took place. At the end of the discussion the emperor said,
If what you say should be true, he will very soon occupy the earth under my feet, and if I knew that I would reach him definitely, I would go immediately to meet Him; and were I with him, then I would certainly wash his feet.
In 629 according to tradition, Muhammad sent a force of 3,000 men to fight 100,000 Byzantines near Al Karak. The Battle of Mu'tah ended when both sides retreated.
Read more about this topic: Muhammad's Views On Christians