Sahih al-Bukhari, Book 97, Hadith 166 (In-Book) ⓘ

And Ibn 'Abbas narrated: Abu Sufyan bin Harb told me that Heraclius called for his translator and then asked for the letter of the Prophet, 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)

وَقَالَ ابْنُ عَبَّاسٍ أَخْبَرَنِي أَبُو سُفْيَانَ بْنُ حَرْبٍ، أَنَّ هِرَقْلَ، دَعَا تَرْجُمَانَهُ، ثُمَّ دَعَا بِكِتَابِ النَّبِيِّ صلى الله عليه وسلم فَقَرَأَهُ ‏"‏ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ مِنْ مُحَمَّدٍ عَبْدِ اللَّهِ وَرَسُولِهِ إِلَى هِرَقْلَ، وَ‏{‏يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَى كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ ‏}‏‏"‏ الآيَةَ‏.‏