Mishkat al-Masabih 3926
Ibn ‘Abbas told that the Prophet wrote to Qaisar summoning him to Islam. He sent Dihya al-Kalbi with his letter to him and ordered him to hand it to the governor of Busra for him to convey to Qaisar. Its contents were:
In the name of God, the Compassionate, the Merciful. From Muhammad, God’s servant and Messenger, to Hiraql1 chief of the Byzantines. Peace be to those who follow the guidance. To proceed: I send you the summons to accept Islam ( bi da'iyat al-islam ). If you accept Islam you will be safe, and if you accept Islam God will bring you your reward twofold; but if you turn away you will be guilty of the sin of your followers ( al-arisiyin ).2 “0 people of the Book, come to a word which is common between us and you, that we should worship only God, not attribute any partner to Him, or take one another as lords apart from God. But if they turn away say, Testify that we are Muslims."3 

    1. Heraclius.
    2. Arisi means a tiller of the soil, but is here used as a general reference to the common people who will follow his example.
    3. Al-Qur’an; 3:64.

(Bukhari and Muslim.) A version by Muslim has “From Muhammad God’s Messenger," “the sin of al-yarisiyin ” and bi-di'ayat al-islam .
عَنِ ابْنِ عَبَّاسٍ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَتَبَ إِلَى قَيْصَرَ يَدْعُوهُ إِلَى الْإِسْلَامِ وَبَعَثَ بِكِتَابِهِ إِلَيْهِ دِحْيَةَ الْكَلْبِيَّ وَأَمَرَهُ أَنْ يَدْفَعَهُ إِلَى عَظِيمِ بُصْرَى لِيَدْفَعَهُ إِلَى قَيْصَرَ فَإِذَا فِيهِ: 
" بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ مِنْ مُحَمَّدٍ عَبْدِ اللَّهِ وَرَسُولِهِ إِلَى هِرَقْلَ عَظِيمِ الرُّومِ سَلَامٌ عَلَى مَنِ اتَّبَعَ الْهُدَى أَمَّا بَعْدُ فَإِنِّي أدْعوكَ بداعيَةِ الْإِسْلَامِ أَسْلِمْ تَسْلَمْ وَأَسْلِمْ يُؤْتِكَ اللَّهُ أَجَرَكَ مَرَّتَيْنِ وَإِنْ تَوَلَّيْتَ فَعَلَيْكَ إِثْمُ الْأَرِيسِيِّينَ وَ (يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَى كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ أَن لَا نَعْبُدَ إِلَّا اللَّهَ وَلَا نُشْرِكَ بِهِ شَيْئًا وَلَا يَتَّخِذَ بَعْضُنَا بَعْضًا أَرْبَابًا مِنْ دُونِ اللَّهِ فَإِنْ تَوَلَّوْا فَقُولُوا: اشْهَدُوا بِأَنَّا مُسْلِمُونَ)
مُتَّفَقٌ عَلَيْهِ. وَفِي رِوَايَةٍ لِمُسْلِمٍ قَالَ:
مِنْ محمَّدٍ رسولِ اللَّهِ " وَقَالَ: «إِثمُ اليريسيِّينَ» وَقَالَ: «بِدِعَايَةِ الْإِسْلَام»