Abu Sufyan bin Harb told me that Heraclius called for his translator and then asked for the letter of the Prophet (ﷺ), and the former read it (thus):

"In the Name of Allah, the Most Gracious, the Merciful. (This letter is) from Muhammad bin 'Abdullah, to Heraclius. "...O people of the Scripture (Jews and Christians): Come to a word that is just between us and you that we worship none but Allah..." (V.3:64)