Sahih al-Bukhari 2940, 2941

Ibrahim ibn Hamza narrated to us; Ibrahim ibn Sa'd narrated to us from Salih ibn Kaysan, from Ibn Shihab, from 'Ubayd Allah ibn 'Abdullah ibn 'Utba, from 'Abdullah ibn 'Abbas (may Allah be pleased with them both), who informed him that the Messenger of Allah (ﷺ) wrote to Caesar, inviting him to Islam. He sent his letter with Dihya al-Kalbi and instructed him to hand it to the governor of Busra, so that the governor could deliver it to Caesar. When Allah had driven the Persian forces away from Caesar, Caesar walked from Hims to Iliya, in gratitude for what Allah had granted him. When the Messenger of Allah's letter reached Caesar, he read it and said, 'Find someone here from his people so that I may ask them about the Messenger of Allah (ﷺ).'

Ibn 'Abbas said: Abu Sufyan informed me that he was in al-Sham with some men of Quraysh who had come as merchants during the truce between the Messenger of Allah (ﷺ) and the disbelievers of Quraysh. Abu Sufyan said: Caesar's envoy found us somewhere in al-Sham and took me and my companions with him until we reached Iliya. We were admitted to Caesar and found him seated in his royal court, wearing a crown, with the Roman dignitaries around him. He said to his interpreter, 'Ask them which of them is closest in kinship to this man who claims to be a prophet.' Abu Sufyan said: I replied, 'I am the closest of them in kinship.' He asked, 'What is your relationship to him?' I said, 'He is my cousin.' At that time, I was the only one in the caravan from Banu 'Abd Manaf. Caesar said, 'Bring him near.' He ordered my companions to stand behind me, beside my shoulders, then said to his interpreter, 'Tell his companions: I am going to ask this man about the one who claims to be a prophet. If he lies, contradict him.' Abu Sufyan said: By Allah, had I not been ashamed that my companions might report a lie from me, I would have lied about him when Caesar asked me. But I was ashamed that they might report a lie from me, so I told the truth.

Then Caesar said to his interpreter, 'Ask him: What is this man's standing among you by lineage?' I replied, 'He is of distinguished lineage among us.' He asked, 'Has anyone among you made this claim before him?' I replied, 'No.' He said, 'Did you accuse him of lying before he made this claim?' I replied, 'No.' He asked, 'Was any of his forefathers a king?' I replied, 'No.' He said, 'Do the eminent people follow him, or the weak?' I replied, 'Rather, the weak.' He asked, 'Are they increasing or decreasing?' I replied, 'Rather, they are increasing.' He asked, 'Does anyone turn back in resentment from his religion after entering it?' I replied, 'No.' He asked, 'Does he betray?' I replied, 'No, but we are now in a truce with him, and we fear that he may betray us.' Abu Sufyan said: I could find no opportunity to insert anything against him except that remark, and I feared it would be reported from me. Caesar asked, 'Have you fought him, or has he fought you?' I replied, 'Yes.' He asked, 'How have the fighting and the outcome between you been?' I said, 'It has alternated: one time the advantage is ours, and another time it is his.' He asked, 'What does he command you to do?' I replied, 'He commands us to worship Allah alone and associate nothing with Him in worship; he forbids us what our forefathers used to worship; and he commands us to pray, give charity, be chaste, keep covenants, and render trusts.'

When I had said that, Caesar told his interpreter, 'Say to him: I asked you about his lineage among you, and you said that he is of distinguished lineage. Such is how messengers are sent among the distinguished lineage of their people. I asked you whether anyone among you had made this claim before him, and you said no. Had someone among you made this claim before him, I would have said that this is a man following a statement made before him. I asked you whether you had accused him of lying before he made this claim, and you said no. So I know that he would not have abandoned lying to people only to lie about Allah. I asked you whether any of his forefathers was a king, and you said no. Had one of his forefathers been a king, I would have said that he seeks the kingdom of his forefathers. I asked you whether the eminent people or the weak follow him, and you said the weak follow him; they are the followers of messengers. I asked you whether they are increasing or decreasing, and you said they are increasing. Such is faith until it is complete. I asked you whether anyone turns back in resentment from his religion after entering it, and you said no. Such is faith: when its joy mingles with hearts, no one resents it. I asked you whether he betrays, and you said no. Such is the way of messengers: they do not betray. I asked you whether you fought him and he fought you, and you said that it happened, and that the outcome between you alternates, with the advantage sometimes yours and sometimes his. Such is the way of messengers: they are tested, and the final outcome is theirs. I asked you what he commands you to do, and you said that he commands you to worship Allah and associate nothing with Him, forbids you what your forefathers used to worship, and commands you to pray, speak truthfully, be chaste, keep covenants, and render trusts.' He said, 'This is the description of a prophet. I knew that he would appear, but I did not think he would be from among you. If what you have said is true, he will soon rule the place beneath these two feet of mine. If I hoped I could reach him, I would go to great lengths to meet him; and if I were with him, I would wash his feet.'

Abu Sufyan said: Caesar then called for the Messenger of Allah's letter, and it was read. It said: 'In the name of Allah, the Most Compassionate, the Most Merciful. From Muhammad, the servant of Allah and His Messenger, to Heraclius, the great ruler of the Romans. Peace be upon whoever follows guidance. To proceed: I invite you with the call of Islam. Submit, and you will be safe; submit, and Allah will give you your reward twice over. But if you turn away, then upon you is the sin of the arisiyyin. O People of the Book, come to a word common between us and you: that we worship none but Allah, associate nothing with Him, and that none of us take others as lords besides Allah. But if they turn away, then say: Bear witness that we are Muslims.'

Abu Sufyan said: When Caesar had finished speaking, the voices of the Roman dignitaries around him rose, and their uproar grew loud; I could not make out what they were saying. We were ordered to leave and were taken out. When I went out with my companions and was alone with them, I said, 'The matter of Ibn Abi Kabsha has become great: this king of the Banu al-Asfar fears him.' Abu Sufyan said: By Allah, I remained humiliated, certain that his cause would prevail, until Allah placed Islam in my heart, even though I disliked it.