According to Insanul Uyun, Seera-e Nabawiyya, Tarikh-e Tabari and Kamil Ibn Athir, when the Caesar received the Holy Prophet's letter he became curious and ordered a man from the Hijaz to be found out and brought to him so that he might make investigations about Muhammad. By chance at that time Abu Sufyan and some other Qurayshites had gone to Syria for the purpose of trade. They were taken to Jerusalem and were presented at the Caesar's court.
He asked them: "Who amongst you is most closely related to the man who claims to be the Prophet of Allah?" Abu Sufyan replied: "I am more closely related to him than all others". It was a fact because none of the Qurayshites in the caravan, except Abu Sufyan, was a descendant of Abd Munaf. Caesar said: "What is your relationship with Muhammad?" Abu Sufyan replied: "He is the son of my uncle". Caesar said: "Come nearer". After this he ordered the companions of Abu Sufyan to stand behind him.
Then he turned to the interpreter and said: "Tell the companions of Abu Sufyan that I have made him sit before them so that I may question him about the man who claims to be the Prophet of Allah and I have made them sit behind him so that if he tells a lie they may not feel ashamed on account of looking him in the face and may contradict him. Abu Sufyan used to say later: "I swear by Allah that if I had not been afraid that if I told a lie those persons would contradict me, I would have told a lie. However, I felt ashamed and spoke the truth against my wish".
The following conversation took place between the Caesar and Abu Sufyan:
Caesar: To what sort of family does this proclaimer of prophet hood belong?
Abu Sufyan: He belongs to a noble family.
Caesar: Did anyone amongst you make such a claim earlier?
Abu Sufyan: No.
Caesar: Did he ever tell a lie before he claimed to be a Prophet?
Abu Sufyan : No.
Caesar: Has there been any king among his ancestors?
Abu Sufyan: No.
Caesar: How is his wisdom, understanding and intelligence?
Abu Sufyan: We have not observed any defect in his wisdom, understanding and intelligence.
Caesar: What kind of people has embraced this religion? The nobles and aristocrats or the middle class and the poor?
Abu Sufyan: The middle class and the poor.
Caesar: Are his followers increasing or decreasing?
Abu Sufyan: They are increasing day after day.
Caesar: Does it so happen that someone of them may hate the religion of Islam and apostatize?
Abu Sufyan: No.
Caesar: Does Muhammad break promises?
Abu Sufyan: We have not seen any such thing from him so far.
We have since concluded a covenant with him and do not know how he will act in future (Abu Sufyan meant the 10-year's covenant concluded at Hudaybiya).
Caesar: Have you fought any battle against him so far?
Abu Sufyan: Yes.
Caesar: What was the result of your fighting?
Abu Sufyan: Sometimes we won and sometimes he was victorious. In the Battle of Badr he was successful against us, but I did not participate in that battle. A year later, (i.e. in the Battle of Uhud) we launched an attack on his city and split the bellies and cut off the ears and noses of the people.
Caesar: What does he order you to do?
Abu Sufyan: He orders us to worship Allah and not to associate anything with Him in the matter of authority .He forbids us to worship the idols which have been worshipped by our ancestors. He asks us to offer prayers, give alms, speak the truth, observe chastity, honor the promises and covenants and not to commit breach of trust".
The Caesar considered the investigation sufficient about the personality of the Holy Prophet. He, therefore, said to Abu Sufyan: I asked you about his ancestry and you replied that he comes from a noble family. All the Prophets of Allah are of noble birth in their tribes. I asked you whether someone from amongst you claimed to be a prophet earlier and you replied in the negative. Of course, if someone from amongst you had made such a claim it might be said that he was imitating what had been said earlier.
Then I asked you whether he had been telling lies before he claimed to be a prophet and you yourself admitted that there had been no such thing and he had never lied to the people. I then concluded that there was no reason why a person who did not lie to the people should calumniate Allah. I asked you whether there had been any king among his ancestors and you replied in the negative. Of course, if there had been any king among his ancestors it might be said that he too had risen to regain kingship.
Then I asked you whether the aristocrats and powerful persons or the poor had embraced his religion and you replied that they were the poor people. And the followers of the Prophets of God are always these poor people (i.e. the penniless and not those, who accumulate wealth by fair and foul means in the prevailing conditions, and oppose everyone, who aims at bringing about a revolution to reform the society).
The Caesar continued: "Thereafter I asked you whether the number of the followers of Muhammad was increasing or decreasing gradually and you replied that their number was increasing day after day. Faith in God and His true Prophets produces such results i.e. it attracts more adherents day after day till it attains perfection. I also asked you whether it so happened that someone from amongst the Muslims has shown hatred against Islam or has turned apostate and you said that there had been no such case.
This is the case with faith, because when one's heart acquires relief and comfort by means of faith it does not feel sad. Then I asked you whether you had ever fought with him and you replied that battles had taken place between you and him, and in some of them you were successful whereas in others he was victorious. That is what happens to the Prophets of God. At times they get into difficulties but eventually their advancement and success is certain.
Then I asked you what he ordered you to do and you replied that he ordered you to offer prayers, give alms, observe piety and chastity, honor your promises and not to commit breach of trust. You have admitted that he has never been dishonest, deceitful and unfaithful. This is how the Prophets of God behave. They are not dishonest, deceitful and unfaithful. In the light of these enquiries I am certain that he is the Prophet of God".
This was an example of how an enemy becomes helpless and humble before a sublime personality and cannot muster up his courage to tamper with the realities.