In the name of Allah the Beneficent and the Merciful
(This letter) is from Muhammad (S), the servant of Allah and His Messenger to Heraclius, the ruler of Byzantine.
Peace be upon him who follows the right path.
Further more I invite you to Islam and if you become a Muslim you will be safe, and Allah will double your reward and if you reject this invitation, you will be committing a sin by misguiding your arisiyin (subjects).
(And I recite to you Allah's statement)
"O people of the scripture! Come to a word common to you and us that we worship none but Allah and that we associate nothing in worship with Him and none of us shall take others as Lords besides Allah. Then if they turn away say: bear witness that we are Muslims" (Qur'an 3:64)