Sahih Bukhari Volume 8, Book 74, Hadith Number 277.

Narated By Abu Sufyan bin Harb : That Heraclius had sent for him to come along with a group of the Quraish who were trading in Sha’m, and they came to him. Then Abu Sufyan mentioned the whole narration and said, “Heraclius asked for the letter of Allah’s Apostle. When the letter was read, its contents were as follows: ‘In the name of Allah, the Beneficent, the Merciful. From Muhammad, Allah’s slave and His Apostle to Heraclius, the Chief of Byzantines: Peace be upon him who follows the right path (guidance)! Amma ba’du (to proceed)…'” (See Hadith No 6, Vol 1 for details)