Al-Mubaarakpoori (a scholar) said
“The reason (for calling the kings) is that if they accept Islam, then their people will follow them therein. During the ending of the sixth year (after migrating from Makkah to Madinah), when the Messenger of Allah, sallallaahu ‘alayhi wa sallam, returned from Hudaybiyyah, he wrote to the kings, inviting them to Islam.”
Ibn Hishaam (a scholar) said: “The Messenger of Allah, sallallaahu ‘alayhi wa sallam, sent messengers from his Companions, and sent letters with them that had been written, calling them to Islam.
He, sallallaahu ‘alayhi wa sallam, sent Dihyah ibn Khaleefah Al-Kalbi to Caesar, the Emperor of Byzantium; `Abdullaah ibn Huthaafah As-Sahmi was sent to Kisraa, the King of Persia; `Amr ibn Umayyah Adh-Dhamri was sent to An-Na jaashi, the King of Ethiopia; and Haatib ibn Abu Balta`ah was sent to Al-Muqawqas, the King of Alexandria.”
In the story of Abu Sufyaan with Heraclius, he said to Abu Sufyaan, “If what you say is true, then he is a Prophet. I knew that he would appear, but had no idea that he would be one of you. If I were able to reach him, I would love to meet him, and if I had been present with him, I would have washed his feet; also, know that his dominion will reach what is beneath my two feet.”
Then he called for the letter of the Messenger of Allah, sallallaahu ‘alayhi wa sallam, and read it, and it said
“In the Name of Allah, The Most Merciful, The Ever Merciful; from Muhammad, the Messenger of Allah, to Heraclius, the Emperor of Byzantium; peace be on those who follow the guidance. To proceed: I call you to accept Islam. Accept Islam and you will be safe; Allah will give you your reward twice, but if you turn away, then you have the burden of the sins of the peasants on your back.”