When the letter of the Messenger of Allah (ﷺ) came to Caesar, he said when he read it: Find me one of his people so that I can ask him about the Messenger of Allah (ﷺ). Ibn 'Abbas (رضي الله عنه) said: Abu Sufyan bin Harb told me that he was in Syria with some men of Quraish who had come for trade, and that was at a time when there was a truce between the Messenger of Allah (ﷺ) and the disbelievers of Quraish. Abu Sufyan said: The messenger of Caesar came to me and took me and my companions to Jerusalem, where we were admitted into his [Caesar's] presence. He was sitting in his royal court, wearing a crown and with the Byzantine leaders around him. He said to his interpreter: Ask them which of them is closest in lineage to this man who claims to be a Prophet. Abu Sufyan said: I am the closest in lineage to him. He said: What is your relationship to him? I said: He is my cousin (the son of my paternal uncle). Abu Sufyan said: On that occasion, there was no one else of Banu 'Abd Manaf in the caravan except me. Caesar said: Let him come close to me. Then he instructed that my companions should be made to stand behind me, at my shoulder. Then he said to his interpreter: Tell his companions that I am going to ask this one about this man who claims to be a Prophet, and if he lies they should say that he is lying. Abu Sufyan said: By Allah, were it not that it would have been shameful at that time for my companions to describe me as a liar, I would have lied when he asked me. But I felt that it was shameful to be described as a liar, so I told the truth about him. Then Caesar said to his interpreter: Say to him: What kind of lineage does this man have among you? I said: He is of a noble lineage among us. He said: Did anyone among you say the same thing before him? I said: No. He said: Did you ever blame him for telling lies before he said what he said? I said: No. He said: Was anyone among his forebears a king? I said: No. He said: Do the nobles of the people follow him, or the poor and weak? I said: The poor and weak (follow him). He said: Are they increasing or decreasing in number? I said: They are increasing. He said: Does anyone leave his religion because he is displeased with it after entering it? I said: No. He said: Does he break his promises? I said: No, but now we have a truce with him and we are afraid that he may break the truce. Abu Sufyan said: Other than that, I could not find anything to say against him to undermine him, and I was afraid that that would be held against me. He said: Did you fight him or did he fight you? I said: Yes. He said: What was the outcome of your battles with him? I said: It varied; sometimes he was victorious and sometimes we were. He said: What does he enjoin you to do? I said: He enjoins us to worship Allah alone and not to associate anything with Him; he tells us not to worship what our fathers worshipped; and he enjoins us to pray, give charity, be chaste, keep promises and render back trusts. He said to his interpreter when I said that to him: Tell him: I asked about his lineage and you said that he has a noble lineage among you; this is how all the Messengers are, they are the nobles of their people. I asked you whether anyone among you had said this before, and you said no. I thought that if anyone among you had said this before, I would have said that he was a man who was following the example of someone who had said something similar before. I asked you whether you had accused him of lying before he said what he said, and you said no. Then I realised that the one who did not tell lies about people would never tell a lie about Allah. I asked you whether any of his forebears had been a king and you said no. I thought that if any of his forebears had been a king. I would have said that he was a man who was seeking his father's kingdom. I asked you whether the nobles among the people follow him or the weak and poor, and you said that the weak and poor follow him; they are always the followers of the Messengers. I asked you whether they were increasing or decreasing in number, and you said that they are increasing. This is how true faith is until it is complete. I asked you whether anyone leaves his religion because he is displeased with it after entering it, and you said no. This is how true faith is when its cheerfulness enters the heart and mixes with it: no one becomes displeased with it. I asked you whether he broke his promises and you said no; this is how the Messengers are. I asked you whether you had fought him or he had fought you and you said that this had happened, and that the outcome of the battles between you and him varied: sometimes he was victorious and sometimes you were. This is how the Messengers are; they are put to trial but the final victory is always theirs. I asked you what he enjoins you to do, and you said that he enjoins you to worship Allah alone and not to associate anything with Him; he tells you not to worship what your forefathers worshipped; and he enjoins you to give in charity, pray, be chaste, fulfill promises and render back trusts. This is the character of a Prophet who I knew would appear, but I did not think that he would be from among you. If what you have said to me is true, he will soon take possession of the land beneath my feet. By Allah, if I knew that I would definitely reach him, I would immediately go to meet him, and if I were with him I would wash his feet. Abu Sufyan said: Then he called for the letter of the Messenger of Allah (ﷺ) and ordered that it be read out loud. In the letter he said: "In the Name of Allah, the Most Gracious, the Most Merciful. From Muhammad, the slave of Allah and His Messenger, to Heraclius the ruler of Byzantium. Peace be upon those who follow true guidance. I am calling you to Islam: become Muslim and you will be safe. Become Muslim and Allah will bestow upon you a double reward, but if you turn away, the sins of the peasants will be upon you. "O people of the Scripture (Jews and Christians): Come to a word that is just between us and you, that we worship none but Allah (Alone), and that we associate no partners with Him, and that none of us shall take others as lords besides Allah. Then, if they turn away, say: "Bear witness that we are Muslims" [Al ‘Imran 3:64]." Abu Sufyan said: When he finished his speech, the voices of the Byzantine leaders who were around him were raised, and there was so much noise that I could not understand what they said, Then he ordered that we be sent away. When I left with my companions and was alone with them, I said to them. The affair of Ibn Abi Kabshah[1] has gained power: this king of Banul-Asfar [the Byzantines] fears him. Abu Sufyan said: By Allah, I became humble after that and was certain that he would be victorious, until Allah instilled Islam in my heart even though I was reluctant.

[1]A derogatory nickname by the Quraish to refer to the Messenger of Allah (ﷺ).