He read it and found:

“In the name of Allah, the Most Merciful, the Bestower of Mercy. From Muhammad, the servant and Messenger of Allah, to Heraclius, the great ruler of Byzantium. Peace be upon those who follow true guidance. To proceed: I invite you with the call of Islam—submit and you will be safe; submit, and Allah will grant you your reward twice over. But if you turn away, the sin of your subjects will rest upon you.

‘O People of the Scripture, come to a word that is fair between us and you: that we worship none but Allah, associate nothing with Him, and that none of us take others as lords alongside Allah. If they turn away, then say, “Bear witness that we are Muslims.”’
[Al ʿImrān: 64]”

Abu Sufyan said:
When he had said what he wished and finished reading the letter, an uproar arose about him, voices grew loud, and we were ushered out. As we left, I said to my companions, “The matter of Ibn Abi Kabshah has become momentous indeed— the king of Bani al-Asfar (the Byzantines) fears him! I remained certain that he would prevail until Allah brought Islam into my heart.”