Sahih Muslim Book 19, Hadith Number 4381.

Chapter : The Holy Prophet’s (may peace be upon him) letter to Hiraql (Caesar) inviting him to Islam.

This hadith has been narrated on the authority of Ibn Shihab with the same chain of transmitters but with the addition: “When Allah inflicted defeat on the armies of Persia, Caesar moved from Hims to Aelia (Bait al-Maqdis) for thanking Allah as He granted him victory.” In this hadith these words occur: “From Muhammad, servant of Allah and His Messenger,” and said: “The sin of your followers,” and also said the words: “to the call of Islam”.