كتاب الجهاد والسير
32  
The Book of Jihad and Expeditions
(26)
Chapter: The Prophet (saws) wrote to Heraclius, the ruler of Syria, inviting him to Islam
(26)
باب كِتَابِ النَّبِيِّ صلى الله عليه وسلم إِلَى هِرَقْلَ يَدْعُوهُ إِلَى الإِسْلاَمِ ‏‏
Sahih Muslim 1773 b

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".
وَحَدَّثَنَاهُ حَسَنٌ الْحُلْوَانِيُّ، وَعَبْدُ بْنُ حُمَيْدٍ، قَالاَ حَدَّثَنَا يَعْقُوبُ، - وَهُوَ ابْنُ إِبْرَاهِيمَ بْنِ سَعْدٍ - حَدَّثَنَا أَبِي، عَنْ صَالِحٍ، عَنِ ابْنِ شِهَابٍ، بِهَذَا الإِسْنَادِ وَزَادَ فِي الْحَدِيثِ وَكَانَ قَيْصَرُ لَمَّا كَشَفَ اللَّهُ عَنْهُ جُنُودَ فَارِسَ مَشَى مِنْ حِمْصَ إِلَى إِيلِيَاءَ شُكْرًا لِمَا أَبْلاَهُ اللَّهُ ‏.‏ وَقَالَ فِي الْحَدِيثِ ‏"‏ مِنْ مُحَمَّدٍ عَبْدِ اللَّهِ وَرَسُولِهِ ‏"‏ ‏.‏ وَقَالَ ‏"‏ إِثْمَ الْيَرِيسِيِّينَ ‏"‏ ‏.‏ وَقَالَ ‏"‏ بِدَاعِيَةِ الإِسْلاَمِ ‏"‏ ‏.‏
Reference : Sahih Muslim 1773b
In-book reference : Book 32, Hadith 90
USC-MSA web (English) reference : Book 19, Hadith 4381
  (deprecated numbering scheme)