After settling down in Medina on return from Hudaibiya, the Prophet(sa) instituted another plan for the spread of his Message. When he mentioned this to the Companions, some of them who were acquainted with the customs and forms observed in the courts of kings told the Prophet(sa) that kings did not entertain letters which did not bear the seals of the senders. Accordingly the Prophet(sa) had a seal made on which were engraved the words, Muhammad Rasulullah(sa).
Out of reverence, Allah was put at the top, beneath it Rasul and lastly Muhammad(sa).
In Muharram 628, envoys went to different capitals, each with a letter from the Prophet(sa), inviting the rulers to accept Islam. Envoys went to Heraclius, the Roman Emperor, the Kings of Iran, Egypt (the King of Egypt was then a vassal of the Kaiser) and Abyssinia. They went to other kings and rulers also. The letter addressed to the Kaiser was taken by Dihya Kalbi(ra) who was instructed to call first on the Governor of Busra. When Dihya(ra) saw the Governor, the great Kaiser himself was in Syria on a tour of the Empire. The Governor readily passed Dihya(ra) on to the Kaiser. When Dihya(ra) entered the court, he was told that whoever was received in audience by the Kaiser must prostrate himself before him. Dihya(ra) refused to do this, saying that Muslims did not bow before any human being. Dihya(ra), therefore, sat before the Kaiser without making the prescribed obeisances. The Kaiser had the letter read by an interpreter and asked if an Arab caravan was in the town. He said he desired to interrogate an Arab about this Arabian Prophet(sa) who had sent him an invitation to accept Islam. It so happened that Abu Sufyan(ra) was in the town with a commercial caravan. The court officials took him to the Kaiser. Abu Sufyan(ra) was ordered to stand in front of the other Arabs, who were told to correct him if he should tell a lie or make a wrong statement. Then Heraclius proceeded to interrogate Abu Sufyan(ra). The conversation is thus recorded in history:
H: Do you know this man who claims to be a Prophet(sa) and who has sent me a letter? Can you say what sort of family he comes from?
A-S: He comes of a noble family and is one of my relations.
H: Have there been Arabs before him who have made claims similar to his?
A-S: No.
H: Did your people ever charge him with lying before he announced his claim?
A-S: No.
H: Has there been a king or a ruler among his forefathers?
A—S: No.
H: How do you judge his general ability and his capacity for judgement?
A—S: We have never found any fault in his ability and his capacity for judgement.
H: What are his followers like?Are they big and powerful persons or are they poor and humble?
A—S: Mostly poor and humble and young.
H: Do their numbers tend to increase or decrease?
A—S: To increase.
H: Do his followers ever go back to their old beliefs?
A—S: No.
H: Has he ever broken a pledge?
A—S: Not so far. But we have recently entered into a new pact with him. Let us see what he does about it.
H: Have you had any fight with him yet?
A—S: Yes.
H: With what result?
A—S: Like buckets on a wheel, victory and defeat alternate between us and him. In the Battle of Badr, for instance, in which I was not present, he was able to overpower our side. In the Battle of Uhud, in which I commanded our side, we took his side to task. We tore their stomachs, their ears and their noses,
H: But what does he teach?
A—S: That we should worship the One God and not set up equals with Him. He preaches against the idols our forefathers worshipped. He wants us, instead, to worship the Only God, to speak the truth only and always to abjure all vicious and corrupt practices. He exhorts us to be good to one another and to keep our covenants and discharge our trusts.
This interesting conversation came to an end then the Kaiser said:
I first asked you about his family and you said he belonged to a noble family. In truth, Prophets always come of noble families. I then asked you if anyone before him had made a similar claim and you said, No. I asked you this question because I thought that if in the recent past some one had made such a claim, then one could say that this Prophet(sa) was imitating that claim. I then asked you whether he had ever been charged with lying before his claim had been announced and you said, No. I inferred from this that a person who does not lie about men will not lie about God. I next asked you if there had been a king among his forefathers and you said, No. From this I understood that his claim could not be a subtle plan for the recovery of the kingdom. I then asked you whether the entrants into his fold were mostly big, prosperous and powerful individuals or poor and weak. And you said in reply, that they were generally poor and weak, not proud and big, and so are the early followers of a Prophet. I then asked you whether his numbers were increasing or decreasing and you said they were increasing. At this I remembered that the followers of a Prophet go on increasing until the Prophet attains his goal. I then asked you if his followers left him out of disgust or disappointment, and you said, No. At this I remembered that the followers of Prophets are usually steadfast. They may fall away for other reasons, but not out of disgust for the faith. I then asked you if there had been fights between you and him and, if so, with what results. And you said that you and his followers were like buckets on a wheel and the Prophets are like that. In the beginning their followers suffer reverses and meet with misfortunes, but in the end they win. I then asked you about what he teaches and you said he teaches the worship of One God, truth-speaking, virtue and the importance of keeping covenants and discharging trusts. I asked you also whether he ever played false, and you said, No. And this is the way of virtuous men. It seems to me, therefore, that his claim to being a Prophet(sa) is true. I was half expecting his appearance in our time, but I did not know he was going to be an Arab. If what you have told me is true, then I think his influence and his dominion will certainly spread over these lands (Bukhari).
The speech unsettled the courtiers who began to blame the King for applauding a Teacher of another community. Protests were raised. The court officials then sent away Abu Sufyan(ra) and his friends. The text of the letter which the Prophet(sa) wrote to the Kaiser is to be found in historical records. It runs as follows:
From Muhammad(sa), the Servant of God and His Messenger. To the Chief of Rome, Heraclius. Whoever treads the path of divine guidance, on him be peace. After this, O King, I invite you to Islam. Become a Muslim. God will protect you from all afflictions, and reward you twice over. But if you deny and refuse to accept this Message, then the sin not only of your own denial, but of the denial of your subjects, will be on your head. "Say, 'O People of the Book! come to a word equal between us and you that we worship none but Allah, and that we associate no partner with Him, and that some of us take not others for lords beside Allah.' But if they turn away, then say, ‘Bear witness that we have submitted to God' " (Zurqani).
The invitation to Islam was an invitation to believe that God is One and that Muhammad(sa) is His Messenger. Where the letter says that if Heraclius becomes a Muslim, he will be rewarded twice over, the reference is to the fact that Islam teaches belief in both Jesus(as) and Muhammad(sa).
It is said that when the letter was presented to the Emperor, some courtiers suggested it should be torn up and thrown away. The letter, they said, was an insult to the Emperor. It did not describe the Emperor as Emperor but only as Sahibul Rum, i.e., the Chief of Rome. The Emperor, however, said that it was unwise to tear up the letter without reading it. He also said that the address, 'Chief of Rome', was not wrong. After all, the Master of everything was God. An Emperor was only a chief.
When the Prophet(sa) was told how his letter had been received by Heraclius, he seemed satisfied and pleased and said that because of the reception which the Roman Emperor had given his letter, his Empire would be saved. The descendants of the Emperor would continue long to rule over the Empire. That is in fact what happened. In the wars which took place later, a large part of the Roman Empire, in accordance with another prophecy of the Prophet(sa) of Islam, passed out of the possession of Rome; yet for six hundred years after this, the dynasty of Heraclius remained established in Constantinople. The Prophet's(sa) letter remained preserved in the State archives for a long time. Ambassadors of the Muslim King, Mansur Qalawun, visited the court of Rome, and were shown the letter deposited in a case. The then Roman Emperor showing the letter said it had been received by a forefather of his from their Prophet(sa) and had been carefully preserved.