Sayyidina Ibn Abbas (RA) reported that Abu Sufyan ibn Harb informed him that when he had gone to Syria on a trade mission with some people, Heraclius sent him message. They presented themselves in his court. Thereafter, Sufyan mentioned the hadith. Heraclius called for the letter of Allah’s Messenger (SAW) and it was read out to him. It said, “In the name of Allah, the Compassionate, the Merciful. From Muhammad the slave of Allah and His Messenger (SAW) to Heraclius, the great of Rome. Peace be on him who follows guidance. To procced!”
[Ahmed 2370, Muslim