It has been narrated on the authority of Ibn Abbas who learnt the tradition personally from Abu Safyan. The latter said:

I went out (on a mercantile venture) during the period (of truce) between me and the Messenger of Allah (ﷺ). While I was in Syria, the letter of the Messenger of Allah (ﷺ) was handed over to Hiraql (Ceasar), the Emperor of Rome (who was on a visit to Jerusalem at that time). The letter was brought by Dihya Kalbi who delivered it to the governor of Busra The governor passed it on to Hiraql, (On receiving the letter), he said: Is there anyone from the people of this man who thinks that he is a prophet. People said: Yes. So, I was called along with a few others from the Quraish. We were admitted to Hiraql and he seated usbefore him. He asked: Which of you has closer kinship with the man who thinks that he is a prophet? Abu Sufyan said: I. So they seated me in front of him and stated my companions behind me. Then, he called his interpreter and said to him: Tell them that I am going to ask this fellow (i. e. Abu Sufyan) about the man who thinks that he is a prophet. It he tells me a lie, then refute him. Abu Sufyan told (the narrator): By God, if there was not the fear that falsehood would be imputed to me I would have lied. (Then) Hiraqi said to his interpreter: Inquire from him about his ancestry, I said: He is of good ancestry among us. He asked: Has there been a king among his ancestors? I said: No. He asked: Did you accuse him of falsehood before he proclaimed his prophethood? I said: No. He asked: Who are his follower people of high status or low status? I said: (They are) of low status. He asked: Are they increasing in number or decreasing? I said. No. they are rather increasing. He asked: Does anyone give up his religion, being dissatisfied with it, after having embraced it? I said: No. He asked: Have you been at war with him? I said: Yes. He asked: How did you fare in that war? I said: The war between us and him has been wavering like a bucket, up at one turn and down at the other (i. e. the victory has been shared between us and him by turns). Sometimes he suffered loss at our hands and sometimes we suffered loss at his (hand). He asked: Has he (ever) violated his covenant? I said: No. but we have recently concluded a peace treaty with him for a petiod and we do not know what he is going to do about it. (Abu Sufyin said on oath that he could not interpolate in this dialogue anything from himself more than these words ) He asked: Did anyone make the proclamation (Of prophethood) before him? I said: No. He (now) said to his interpreter: Tell him, I asked him about his ancestry and he had replied that he had the best ancestry. This is the case with Prophets; they are the descendants of the noblest among their people (Addressing Abu Sufyan), he continued: I asked you if there had been a king among his ancestors. You said that there had been none. If there had been a king among his ancestors, I would have said that he was a man demanding his ancestral kingdom. I asked you about his followers whether they were people of high or low status, and you said that they were of rather low status. Such are the followers of the Prophets. I asked you whether you used to accuse him of falsehood before he proclaimed his prophethood, and you said that you did not. So I have understood that when he did not allow himself to tell a lie about the poeple, he would never go to the length of forging a falsehood about Allah. I asked you whether anyone renounced his religion being dissatisfied with it after he had embraced it, and you replied in the negative. Faith is like this when it enters the depth of the heart (it perpetuates them). I asked you whether his followers were increasing or decreasing. You said they were increasing. Faith is like this until it reaches its consummation. I asked you whether you had been at war with him, and you replied that you had been and that the victory between you and him had been shared by turns, sometimes he suffering loss at your hand and sometimes you suffering lost at his. This is how the Prophets are tried before the final victory its theirs. I asked you whether he (ever) violated his covenant, and you said that he did not. This is how the Prophets behave. They never violate (their covenants). I asked you whether anyone before him had proclaimed the same thing, and you replied in the negative. I said: If anyone had made the same proclamation before, I would have thought that he was a man following what had been proclaimed before. (Then) he asked: What does he enjoin upon you? I said: He exhorts us to offer Salat, to pay Zakat, to show due regard to kinship and to practise chastity. He said: It what you have told about him is true, he is certainly a Prophet. I knew that he was to appear but I did not think that he would be from among you. If I knew that I would be able to reach him. I would love to meet him; and it I had been with him. I would have washed his feet (out of reverence). His dominion would certainly extend to this place which is under my feet. Then he called for the letter of the Messenger of Allah (may pface be upon him) and read it. The letter ran as follows:" In the name of Allah, Most Gracious and Most Merciful. From Muhammad, the Messenger of Allah, to Hiraql, the Emperor of the Romans. Peace be upon him who follows the guidance. After this, I extend to you the invitation to accept Islam. Embrace Islam and you will be safe. Accept Islam, God will give you double the reward. And if you turn away, upon you will be the sin of your subjects." O People of the Book, come to the word that is common between us that we should worship none other than Allah, should not ascribe any partner to Him and some of us should not take their fellows as Lords other than Allah. If they turn away, you should say that we testify to our being Muslims [iii. 64]." When he hid finished the reading of the letter, noise and confused clamour was raise around him, and he ordered us to leave. Accordingly, we left. (Addressing my companions) while we were coming out (of the place). I said: Ibn Abu Kabsha (referring sarcastically to the Holy Prophet) has come to wield a great power. Lo! (even) the king of the Romans is afraid of him. I continued to believe that the authority of the Messenger of Allah (ﷺ) would triumph until God imbued me with (the spirit of) Islam.
حَدَّثَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ الْحَنْظَلِيُّ، وَابْنُ أَبِي عُمَرَ، وَمُحَمَّدُ بْنُ رَافِعٍ، وَعَبْدُ بْنُ، حُمَيْدٍ - وَاللَّفْظُ لاِبْنِ رَافِعٍ - قَالَ ابْنُ رَافِعٍ وَابْنُ أَبِي عُمَرَ حَدَّثَنَا وَقَالَ الآخَرَانِ، أَخْبَرَنَا عَبْدُ الرَّزَّاقِ، أَخْبَرَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، عَنْ عُبَيْدِ اللَّهِ بْنِ عَبْدِ اللَّهِ بْنِ عُتْبَةَ، عَنِ ابْنِ عَبَّاسٍ، أَنَّ أَبَا سُفْيَانَ، أَخْبَرَهُ مِنْ، فِيهِ إِلَى فِيهِ قَالَ انْطَلَقْتُ فِي الْمُدَّةِ الَّتِي كَانَتْ بَيْنِي وَبَيْنَ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ فَبَيْنَا أَنَا بِالشَّأْمِ إِذْ جِيءَ بِكِتَابٍ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم إِلَى هِرَقْلَ يَعْنِي عَظِيمَ الرُّومِ - قَالَ - وَكَانَ دِحْيَةُ الْكَلْبِيُّ جَاءَ بِهِ فَدَفَعَهُ إِلَى عَظِيمِ بُصْرَى فَدَفَعَهُ عَظِيمُ بُصْرَى إِلَى هِرَقْلَ فَقَالَ هِرَقْلُ هَلْ هَا هُنَا أَحَدٌ مِنْ قَوْمِ هَذَا الرَّجُلِ الَّذِي يَزْعُمُ أَنَّهُ نَبِيٌّ قَالُوا نَعَمْ - قَالَ - فَدُعِيتُ فِي نَفَرٍ مِنْ قُرَيْشٍ فَدَخَلْنَا عَلَى هِرَقْلَ فَأَجْلَسَنَا بَيْنَ يَدَيْهِ فَقَالَ أَيُّكُمْ أَقْرَبُ نَسَبًا مِنْ هَذَا الرَّجُلِ الَّذِي يَزْعُمُ أَنَّهُ نَبِيٌّ فَقَالَ أَبُو سُفْيَانَ فَقُلْتُ أَنَا ‏.‏ فَأَجْلَسُونِي بَيْنَ يَدَيْهِ وَأَجْلَسُوا أَصْحَابِي خَلْفِي ثُمَّ دَعَا بِتَرْجُمَانِهِ فَقَالَ لَهُ قُلْ لَهُمْ إِنِّي سَائِلٌ هَذَا عَنِ الرَّجُلِ الَّذِي يَزْعُمُ أَنَّهُ نَبِيٌّ فَإِنْ كَذَبَنِي فَكَذِّبُوهُ ‏.‏ قَالَ فَقَالَ أَبُو سُفْيَانَ وَايْمُ اللَّهِ لَوْلاَ مَخَافَةَ أَنْ يُؤْثَرَ عَلَىَّ الْكَذِبُ لَكَذَبْتُ ‏.‏ ثُمَّ قَالَ لِتَرْجُمَانِهِ سَلْهُ كَيْفَ حَسَبُهُ فِيكُمْ قَالَ قُلْتُ هُوَ فِينَا ذُو حَسَبٍ قَالَ فَهَلْ كَانَ مِنْ آبَائِهِ مَلِكٌ قُلْتُ لاَ ‏.‏ قَالَ فَهَلْ كُنْتُمْ تَتَّهِمُونَهُ بِالْكَذِبِ قَبْلَ أَنْ يَقُولَ مَا قَالَ قُلْتُ لاَ ‏.‏ قَالَ وَمَنْ يَتَّبِعُهُ أَشْرَافُ النَّاسِ أَمْ ضُعَفَاؤُهُمْ قَالَ قُلْتُ بَلْ ضُعَفَاؤُهُمْ ‏.‏ قَالَ أَيَزِيدُونَ أَمْ يَنْقُصُونَ قَالَ قُلْتُ لاَ بَلْ يَزِيدُونَ ‏.‏ قَالَ هَلْ يَرْتَدُّ أَحَدٌ مِنْهُمْ عَنْ دِينِهِ بَعْدَ أَنْ يَدْخُلَ فِيهِ سَخْطَةً لَهُ قَالَ قُلْتُ لاَ ‏.‏ قَالَ فَهَلْ قَاتَلْتُمُوهُ قُلْتُ نَعَمْ ‏.‏ قَالَ فَكَيْفَ كَانَ قِتَالُكُمْ إِيَّاهُ قَالَ قُلْتُ تَكُونُ الْحَرْبُ بَيْنَنَا وَبَيْنَهُ سِجَالاً يُصِيبُ مِنَّا وَنُصِيبُ مِنْهُ ‏.‏ قَالَ فَهَلْ يَغْدِرُ قُلْتُ لاَ ‏.‏ وَنَحْنُ مِنْهُ فِي مُدَّةٍ لاَ نَدْرِي مَا هُوَ صَانِعٌ فِيهَا ‏.‏ قَالَ فَوَاللَّهِ مَا أَمْكَنَنِي مِنْ كَلِمَةٍ أُدْخِلُ فِيهَا شَيْئًا غَيْرَ هَذِهِ ‏.‏ قَالَ فَهَلْ قَالَ هَذَا الْقَوْلَ أَحَدٌ قَبْلَهُ قَالَ قُلْتُ لاَ ‏.‏ قَالَ لِتَرْجُمَانِهِ قُلْ لَهُ إِنِّي سَأَلْتُكَ عَنْ حَسَبِهِ فَزَعَمْتَ أَنَّهُ فِيكُمْ ذُو حَسَبٍ وَكَذَلِكَ الرُّسُلُ تُبْعَثُ فِي أَحْسَابِ قَوْمِهَا ‏.‏ وَسَأَلْتُكَ هَلْ كَانَ فِي آبَائِهِ مَلِكٌ فَزَعَمْتَ أَنْ لاَ ‏.‏ فَقُلْتُ لَوْ كَانَ مِنْ آبَائِهِ مَلِكٌ قُلْتُ رَجُلٌ يَطْلُبُ مُلْكَ آبَائِهِ ‏.‏ وَسَأَلْتُكَ عَنْ أَتْبَاعِهِ أَضُعَفَاؤُهُمْ أَمْ أَشْرَافُهُمْ فَقُلْتَ بَلْ ضُعَفَاؤُهُمْ وَهُمْ أَتْبَاعُ الرُّسُلِ ‏.‏ وَسَأَلْتُكَ هَلْ كُنْتُمْ تَتَّهِمُونَهُ بِالْكَذِبِ قَبْلَ أَنْ يَقُولَ مَا قَالَ فَزَعَمْتَ أَنْ لاَ ‏.‏ فَقَدْ عَرَفْتُ أَنَّهُ لَمْ يَكُنْ لِيَدَعَ الْكَذِبَ عَلَى النَّاسِ ثُمَّ يَذْهَبَ فَيَكْذِبَ عَلَى اللَّهِ ‏.‏ وَسَأَلْتُكَ هَلْ يَرْتَدُّ أَحَدٌ مِنْهُمْ عَنْ دِينِهِ بَعْدَ أَنْ يَدْخُلَهُ سَخْطَةً لَهُ فَزَعَمْتَ أَنْ لاَ ‏.‏ وَكَذَلِكَ الإِيمَانُ إِذَا خَالَطَ بَشَاشَةَ الْقُلُوبِ ‏.‏ وَسَأَلْتُكَ هَلْ يَزِيدُونَ أَوْ يَنْقُصُونَ فَزَعَمْتَ أَنَّهُمْ يَزِيدُونَ وَكَذَلِكَ الإِيمَانُ حَتَّى يَتِمَّ ‏.‏ وَسَأَلْتُكَ هَلْ قَاتَلْتُمُوهُ فَزَعَمْتَ أَنَّكُمْ قَدْ قَاتَلْتُمُوهُ فَتَكُونُ الْحَرْبُ بَيْنَكُمْ وَبَيْنَهُ سِجَالاً يَنَالُ مِنْكُمْ وَتَنَالُونَ مِنْهُ ‏.‏ وَكَذَلِكَ الرُّسُلُ تُبْتَلَى ثُمَّ تَكُونُ لَهُمُ الْعَاقِبَةُ وَسَأَلْتُكَ هَلْ يَغْدِرُ فَزَعَمْتَ أَنَّهُ لاَ يَغْدِرُ ‏.‏ وَكَذَلِكَ الرُّسُلُ لاَ تَغْدِرُ ‏.‏ وَسَأَلْتُكَ هَلْ قَالَ هَذَا الْقَوْلَ أَحَدٌ قَبْلَهُ فَزَعَمْتَ أَنْ لاَ ‏.‏ فَقُلْتُ لَوْ قَالَ هَذَا الْقَوْلَ أَحَدٌ قَبْلَهُ قُلْتُ رَجُلٌ ائْتَمَّ بِقَوْلٍ قِيلَ قَبْلَهُ ‏.‏ قَالَ ثُمَّ قَالَ بِمَ يَأْمُرُ كُمْ قُلْتُ يَأْمُرُنَا بِالصَّلاَةِ وَالزَّكَاةِ وَالصِّلَةِ وَالْعَفَافِ قَالَ إِنْ يَكُنْ مَا تَقُولُ فِيهِ حَقًّا فَإِنَّهُ نَبِيٌّ وَقَدْ كُنْتُ أَعْلَمُ أَنَّهُ خَارِجٌ وَلَمْ أَكُنْ أَظُنُّهُ مِنْكُمْ وَلَوْ أَنِّي أَعْلَمُ أَنِّي أَخْلُصُ إِلَيْهِ لأَحْبَبْتُ لِقَاءَهُ وَلَوْ كُنْتُ عِنْدَهُ لَغَسَلْتُ عَنْ قَدَمَيْهِ وَلَيَبْلُغَنَّ مُلْكُهُ مَا تَحْتَ قَدَمَىَّ ‏.‏ قَالَ ثُمَّ دَعَا بِكِتَابِ رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَرَأَهُ فَإِذَا فِيهِ ‏"‏ بِسْمِ اللَّهِ الرَّحْمَنِ الرَّحِيمِ مِنْ مُحَمَّدٍ رَسُولِ اللَّهِ إِلَى هِرَقْلَ عَظِيمِ الرُّومِ سَلاَمٌ عَلَى مَنِ اتَّبَعَ الْهُدَى أَمَّا بَعْدُ فَإِنِّي أَدْعُوكَ بِدِعَايَةِ الإِسْلاَمِ أَسْلِمْ تَسْلَمْ وَأَسْلِمْ يُؤْتِكَ اللَّهُ أَجْرَكَ مَرَّتَيْنِ وَإِنْ تَوَلَّيْتَ فَإِنَّ عَلَيْكَ إِثْمَ الأَرِيسِيِّينَ وَ ‏{‏ يَا أَهْلَ الْكِتَابِ تَعَالَوْا إِلَى كَلِمَةٍ سَوَاءٍ بَيْنَنَا وَبَيْنَكُمْ أَنْ لاَ نَعْبُدَ إِلاَّ اللَّهَ وَلاَ نُشْرِكَ بِهِ شَيْئًا وَلاَ يَتَّخِذَ بَعْضُنَا بَعْضًا أَرْبَابًا مِنْ دُونِ اللَّهِ فَإِنْ تَوَلَّوْا فَقُولُوا اشْهَدُوا بِأَنَّا مُسْلِمُونَ‏}‏ فَلَمَّا فَرَغَ مِنْ قِرَاءَةِ الْكِتَابِ ارْتَفَعَتِ الأَصْوَاتُ عِنْدَهُ وَكَثُرَ اللَّغْطُ وَأَمَرَ بِنَا فَأُخْرِجْنَا ‏.‏ قَالَ فَقُلْتُ لأَصْحَابِي حِينَ خَرَجْنَا لَقَدْ أَمِرَ أَمْرُ ابْنِ أَبِي كَبْشَةَ إِنَّهُ لَيَخَافُهُ مَلِكُ بَنِي الأَصْفَرِ - قَالَ - فَمَا زِلْتُ مُوقِنًا بِأَمْرِ رَسُولِ اللَّهِ صلى الله عليه وسلم أَنَّهُ سَيَظْهَرُ حَتَّى أَدْخَلَ اللَّهُ عَلَىَّ الإِسْلاَمَ ‏.‏
Reference : Sahih Muslim 1773 a
In-book reference : Book 32, Hadith 89
USC-MSA web (English) reference : Book 19, Hadith 4380
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