This was a hadith in Bukhari telling about the conversation of Abu Sufyan (who was pagan who kept on fighting Islam at that time, then he converted after), when he went to Syria for trade and there he met Heraclius. Here is the hadith from Al Bukhari.
Volume 1, Book 1, Number 6:
Narrated ‘Abdullah bin ‘Abbas:
Abu Sufyan bin Harb informed me that Heraclius had sent a messenger to him while he had been accompanying a caravan from Quraish. They were merchants doing business in Sham (Syria, Palestine, Lebanon and Jordan), at the time when Allah’s Apostle had truce with Abu Sufyan and Quraish infidels. So Abu Sufyan and his companions went to Heraclius at Ilya (Jerusalem). Heraclius called them in the court and he had all the senior Roman dignitaries around him. He called for his translator who, translating Heraclius’s question said to them, “Who amongst you is closely related to that man who claims to be a Prophet?” Abu Sufyan replied, “I am the nearest relative to him (amongst the group).”
Heraclius said, “Bring him (Abu Sufyan) close to me and make his companions stand behind him.” Abu Sufyan added, Heraclius told his translator to tell my companions that he wanted to put some questions to me regarding that man (The Prophet) and that if I told a lie they (my companions) should contradict me.” Abu Sufyan added, “By Allah! Had I not been afraid of my companions labeling me a liar, I would not have spoken the truth about the Prophet. The first question he asked me about him was:
‘What is his family status amongst you?’
I replied, ‘He belongs to a good (noble) family amongst us.’
Heraclius further asked, ‘Has anybody amongst you ever claimed the same (i.e. to be a Prophet) before him?’
I replied, ‘No.’
He said, ‘Was anybody amongst his ancestors a king?’
I replied, ‘No.’
Heraclius asked, ‘Do the nobles or the poor follow him?’
I replied, ‘It is the poor who follow him.’
He said, ‘Are his followers increasing decreasing (day by day)?’
I replied, ‘They are increasing.’
He then asked, ‘Does anybody amongst those who embrace his religion become displeased and renounce the religion afterwards?’
I replied, ‘No.’
Heraclius said, ‘Have you ever accused him of telling lies before his claim (to be a Prophet)?’
I replied, ‘No. ‘
Heraclius said, ‘Does he break his promises?’
I replied, ‘No. We are at truce with him but we do not know what he will do in it.’ I could not find opportunity to say anything against him except that.
Heraclius asked, ‘Have you ever had a war with him?’
I replied, ‘Yes.’
Then he said, ‘What was the outcome of the battles?’
I replied, ‘Sometimes he was victorious and sometimes we.’
Heraclius said, ‘What does he order you to do?’
I said, ‘He tells us to worship Allah and Allah alone and not to worship anything along with Him, and to renounce all that our ancestors had said. He orders us to pray, to speak the truth, to be chaste and to keep good relations with our Kith and kin.’
Heraclius asked the translator to convey to me the following, I asked you about his family and your reply was that he belonged to a very noble family. In fact all the Apostles come from noble families amongst their respective peoples. I questioned you whether anybody else amongst you claimed such a thing, your reply was in the negative. If the answer had been in the affirmative, I would have thought that this man was following the previous man’s statement. Then I asked you whether anyone of his ancestors was a king. Your reply was in the negative, and if it had been in the affirmative, I would have thought that this man wanted to take back his ancestral kingdom.
I further asked whether he was ever accused of telling lies before he said what he said, and your reply was in the negative. So I wondered how a person who does not tell a lie about others could ever tell a lie about Allah. I, then asked you whether the rich people followed him or the poor. You replied that it was the poor who followed him. And in fact all the Apostle have been followed by this very class of people. Then I asked you whether his followers were increasing or decreasing. You replied that they were increasing, and in fact this is the way of true faith, till it is complete in all respects. I further asked you whether there was anybody, who, after embracing his religion, became displeased and discarded his religion. Your reply was in the negative, and in fact this is (the sign of) true faith, when its delight enters the hearts and mixes with them completely. I asked you whether he had ever betrayed. You replied in the negative and likewise the Apostles never betray. Then I asked you what he ordered you to do. You replied that he ordered you to worship Allah and Allah alone and not to worship any thing along with Him and forbade you to worship idols and ordered you to pray, to speak the truth and to be chaste. If what you have said is true, he will very soon occupy this place underneath my feet and I knew it (from the scriptures) that he was going to appear but I did not know that he would be from you, and if I could reach him definitely, I would go immediately to meet him and if I were with him, I would certainly wash his feet.’
Heraclius then asked for the letter addressed by Allah’s Apostle which was delivered by Dihya to the Governor of Busra, who forwarded it to Heraclius to read. The contents of the letter were as follows: “In the name of Allah the Beneficent, the Merciful (This letter is) from Muhammad the slave of Allah and His Apostle to Heraclius the ruler of Byzantine. Peace be upon him, who follows the right path. Furthermore I invite you to Islam, and if you become a Muslim you will be safe, and Allah will double your reward, and if you reject this invitation of Islam you will be committing a sin by misguiding your Arisiyin (peasants). (And I recite to you Allah’s Statement:)
‘O people of the scripture! Come to a word common to you and us that we worship none but Allah and that we associate nothing in worship with Him, and that none of us shall take others as Lords beside Allah. Then, if they turn away, say: Bear witness that we are Muslims (those who have surrendered to Allah).’ (3:64).
Abu Sufyan then added, “When Heraclius had finished his speech and had read the letter, there was a great hue and cry in the Royal Court. So we were turned out of the court. I told my companions that the question of Ibn-Abi-Kabsha) (the Prophet Muhammad) has become so prominent that even the King of Bani Al-Asfar (Byzantine) is afraid of him. Then I started to become sure that he (the Prophet) would be the conqueror in the near future till I embraced Islam (i.e. Allah guided me to it).”Follow me onby