Sahih al-Bukhari 6260
Narrated Abu Sufyan bin Harb:
that Heraclius had sent for him to come along with a group of the Quraish who were trading in Sha'm, 
and they came to him. Then Abu Sufyan mentioned the whole narration and said, "Heraclius asked for 
the letter of Allah's Messenger (ﷺ) . When the letter was read, its contents were as follows: 'In the name of 
Allah, the Beneficent, the Merciful. From Muhammad, Allah's slave and His Apostle to Heraclius, the 
Chief of Byzantines: Peace be upon him who follows the right path (guidance)! Amma ba'du (to 
proceed )...' (See Hadith No 6, Vol 1 for details)
حَدَّثَنَا مُحَمَّدُ بْنُ مُقَاتِلٍ أَبُو الْحَسَنِ، أَخْبَرَنَا عَبْدُ اللَّهِ، أَخْبَرَنَا يُونُسُ، عَنِ الزُّهْرِيِّ، قَالَ أَخْبَرَنِي عُبَيْدُ اللَّهِ بْنُ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، أَنَّ ابْنَ عَبَّاسٍ، أَخْبَرَهُ أَنَّ أَبَا سُفْيَانَ بْنَ حَرْبٍ أَخْبَرَهُ أَنَّ هِرَقْلَ أَرْسَلَ إِلَيْهِ فِي نَفَرٍ مِنْ قُرَيْشٍ وَكَانُوا تِجَارًا بِالشَّأْمِ، فَأَتَوْهُ فَذَكَرَ الْحَدِيثَ قَالَ ثُمَّ دَعَا بِكِتَابِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقُرِئَ فَإِذَا فِيهِ ‏
"‏ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ، مِنْ مُحَمَّدٍ عَبْدِ اللَّهِ وَرَسُولِهِ إِلَى هِرَقْلَ عَظِيمِ الرُّومِ، السَّلاَمُ عَلَى مَنِ اتَّبَعَ الْهُدَى، أَمَّا بَعْدُ ‏"‏‏.‏