Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) wrote to Heraclius, the ruler of Rome: "Peace be upon whoever follows the guidance."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20430
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) wrote to Caesar inviting him to Islam, and sent his letter to him with Dihyah al-Kalbi (may Allah be pleased with him). He mentioned the hadith, and in it is that Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: Abu Sufyan (may Allah be pleased with him) informed me, and in it is (his being summoned to Heraclius and being questioned by him). Abu Sufyan (may Allah be pleased with him) said: Then he called for the letter of the Prophet (peace and blessings be upon him), and it was read before him. In it was: "In the name of Allah, the Most Gracious, the Most Merciful. From Muhammad, the servant of Allah and His Messenger, to Heraclius, the ruler of Rome." "Peace be upon whoever follows the guidance."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20431