Emperor of the Eastern Roman Empire (Byzantine Empire). The Muslim traditions recorded a story of Heraclius receiving a letter of invitation to Islam from Muhamamd. At that receipt, he met with Abu Sufyan, then an enemy of Muhammad, who became a Muslim after that. When the Muslims came to conquer Syria, the Muslims were very motivated by rumors that Heraclius had accepted Islam.
An examination of Muslim tradition on Heraclius' response to Muhammad's invitation examines this event in detail.