Abu Sufyan bin Harb reported that Heraclius sent for him while he was with a group from Quraysh who were trading in Sham. They came to him, and he mentioned the hadith, saying, "Then he called for the letter of the Messenger of Allah ﷺ and it was read aloud. It contained the following:In the name of Allah, the Most Merciful, the Most Compassionate. From Muhammad, the servant of Allah and His Messenger, to Heraclius, the great ruler of the Romans. Peace be upon whoever follows the guidance. To proceed."
أن أبا سفيان بن حرب أخبره: أن هرقل أرسل إليه في نفر من قريش، وكانوا تجارا بالشام، فأتوه- فذكر الحديث- قال: ثم دعا بكتاب رسول الله ﷺ فقرئ، فإذا فيه: بسم الله الرحمن الرحيم، من محمد عبد الله ورسوله، إلى هرقل عظيم الروم، السلام على من اتبع الهدى، أما بعد.
Bukhari narrated it in the Book of Seeking Permission (6260), and Muslim in the Book of Jihad and Expeditions (1773), both through the route of Zuhri, who said, Ubaydullah bin Abdullah bin Utba informed me that Ibn Abbas informed him that Abu Sufyan bin Harb informed him, and he mentioned it. This abridged version is Bukhari's.