The Prophet (peace be upon him) took advantage of this period of peace with the Quraish and sent letters to various rulers, including the emperors of the two super-powers of the time, Persia and Byzantium, inviting them to Islam.
When Heraclius, the Byzantine emperor, received the letter, he sent his men to find out all they can about the Prophet (peace be upon him). Abu Sufyan happened to be in Palestine on a business trip and was presented to the King.
Heraclius asked several questions to Abu Sufyan and after listening to his answers, remarked:
“I asked you about his family and your reply was that he belonged to a very noble family. In fact, all the Prophet’s come from noble families. I questioned you whether anybody else amongst you claimed such a thing, your reply was in the negative. If the answer had been yes, I would have thought that this man was copying someone before him. Then I asked you, whether anyone of his family was a king. Your reply was in the negative, and if it had been yes, I would have thought that this man wanted to take back his family’s kingdom.
I further asked whether he was ever accused of telling lies, and your reply was in the negative. I wondered, how a person who does not tell a lie about others could ever tell a lie about God. I then asked you whether the rich people followed him or the poor. You replied that it was the poor who followed him. In fact, all the Prophets have been followed by this very class of people.
Then I asked you whether his followers were increasing or decreasing in number. You replied that they were increasing, and in fact, this is the way of true faith. I further asked you whether there was anybody, who, after embracing his religion, became displeased and left his religion. Your reply was in the negative, and in fact, this is the sign of true faith. Then I asked you what he ordered you to do. You replied that he ordered you to worship God alone and not to worship any thing along with Him, and that ordered you to speak the truth.
If what you have said is true, he will very soon occupy this place underneath my feet. I knew from the scriptures, that he was going to come, but I did not know that he would be from you. If I could reach him, I would go immediately to meet him and if I were with him, I would certainly wash his feet.'