Hz. Dihya was from Madinah. His real name was "Dihya bin Khalifa". However, he was famous for his name "Dihyatul-Kalbi". He was the person with the most beautiful face among the Companions. Gabriel (Jibril) came to the Prophet disguised as Dihya several times. When the Companions saw him, they could not tell whether he was Dihya or Jibril.
Dihya was engaged in trade. Before he became a Muslim, he loved the Messenger of Allah and would bring him a present whenever he came to visit him. However, the Prophet would say to him, "If you really want me to be pleased, be a Muslim and get rid of the fire of Hell." He invited him to Islam.
It was after the Battle of Badr. Jibril gave the Prophet the good news that Dihya would be a Muslim. Soon, Dihya arrived. The Prophet took off his cardigan and gave it to Dihya so that he would sit on it to compliment him. Dihya took the cardigan, kissed it, folded and put it on his head. Then, he became a Muslim by uttering kalima ash-shahada.
From then on, Dihya joined all of the battles with the Prophet. In the 7th year of the Migration, the Prophet sent him as an envoy to the Byzantine Emperor Heraclius. He invited this emperor to Islam. Dihya had the letter of invitation from the Prophet with him.
Dihya had already traveled to foreign countries several times before; so, he knew how to act in other countries very well. He was conscious of the importance of the duty given to him and he knew that he had to be very careful. It was up to him for many people to become Muslims or to reject Islam. For this reason, he had to act cautiously.
After a long and exhausting journey, Hz. Dihya arrived in Jerusalem, where the emperor lived. The people of Jerusalem gave him some advice before entering into the presence of the emperor: "As soon as you enter into the presence of the emperor and see him, prostrate before him and do not lift your head until he gives you permission."
Dihya prostrated only before Allah after he became a Muslim. It was impossible for him to prostrate before anyone except Him. "I can never do this! I will not prostrate before anyone but Allah. They said that the emperor would not accept his letter if he did not do so but Dihya did not agree to do it. Thereupon, they gave him the following advice: "Put your letter on the throne of the emperor. When the emperor takes the letter, he will call the owner of the letter." Hz. Dihya accepted it. He said, "It is all right." He did as they advised him. After a while, the emperor took the letter. When he saw that it was in Arabic, he summoned a translator and had it read. Then he called Abu Sufyan, who was there at that time, to his presence. He asked him several questions about the Prophet. Abu Sufyan had not become a Muslim at that time. However, he answered all of he questions of the emperor correctly. He stated the following regarding the issue later:
“By Allah! If I had not feared that my friends would talk about my lies outside I would definitely have told lies about him.”
After each answer he received to his questions, Heraclius said, "Prophets are like this." He definitely believed that the person who sent the letter was "the last prophet".
When Dihya saw that the heart of Heraclius warmed toward Islam, he became hopeful. He asked him to accept Islam. He knew the way of conveying the message well. Besides, the Messenger of Allah had appointed him due to his ability. He said:
"The one who sent me to you is better than you. Listen to my words humbly. Accept the advice given to you. If you do so, you will understand the advice. If you do not accept the advice, you will not be just."
Then he asked, "Did the Messiah Jesus pray?" The emperor said, "Yes." Dihya said, "Then, I invite you to believe in Allah, to whom the Messiah prayed. I invite you to believe in Allah, who created and governed the heavens and the earth before the Messiah was born of his mother. I invite you to believe in the illiterate Prophet, whose advent Moses and Jesus announced and gave glad tidings of his coming. If you know something regarding the issue and want to attain bliss in the world and the hereafter, remember them. Otherwise, you will lose your bliss in the hereafter. You will remain in the darkness of polytheism and denial in the world. Know it very well that Allah, who is your Lord, destroys dictators and changes boons."
Heraclius believed in the Prophet but he could not declare his belief because he was afraid that he would lose his reign. He expressed this with the following words:
“May Allah give you goodness and make you attain His mercy! I know well that the person who sent you here is a prophet sent by Allah. We were already waiting for him to come. We found his name and attributes written in our books. However, I fear from Greeks about my life! If I were sure of my people, I would follow him and serve him by enduring all kinds of difficulties. Go to Daghatir now. He is one of the greatest Christian scholars. Invite him to Islam.”
The Prophet had already written a letter addressing Usquf Daghatir. Dihya went to Daghatir without losing time. He gave the letter of the Prophet to Daghatir. The Prophet invited Daghatir to Islam in his letter and stated the following:
“Peace be upon those who believe! There is no doubt that Jesus, the son of Mary, is the spirit and word of Allah that He blew into Mary. I believe in Allah and the revelation given to us and to Abraham, Ismail, Isaac, Jacob, and the Tribes and that given to Moses and Jesus and that given to (all) Prophets from their Lord. We make no difference among them. We believe that all of them are prophets. We are Muslims who obey Allah’s orders. May peace be upon the guided ones!”
Usquf Daghatir was a scholar and the chief bishop of Byzantium. He learned from his books that the last prophet would emerge; and he read his attributes. As soon as he read the letter, he believed without hesitation by saying, "I swear by Allah that your master is the prophet sent by Allah. We knew his name and his attributes." Then, he entered a room, took off his black clothes and put on a white garment. There was no room left for black clothes and black thoughts any more. It was necessary to wear white clothes and have bright ideas.
The bishop retired into his house and did not come out. Dihya did not leave him alone; he visited the bishop frequently. The chief bishop called "Usquf Daghatir" did not attend the service in the church that Sunday. In the meantime, the people became aware of this change in the bishop.
The angry and bigoted Greeks surrounded the bishop's house, urging him to address them by shouting with furiously and violently. However, Usquf Daghatir was no longer their bishop; he was a member of the ummah of the Prophet of the end of time. He was probably having his last talk with his close friends, Dihyatul-Kalbi.
Usquf gave the letter that he had written to the Messenger of Allah (pbuh) to Dihya:
"Take this letter, go to our master and give him my regards. Tell him that I have witnessed that there is no god but Allah and that Muhammad is the Messenger of Allah. I have believed in him; I have affirmed him, and I am subject to him. However, as you see, these people are denying this. Tell him exactly what you saw."
The angry crowd on the outside understood that Daghatir was talking to Dihya. They yelled:
"Either he will come out or we will come in. We have seen the change in you since this Arab came and we do not like you."
After sending Dihya, Usquf Daghatir appeared before the people wearing white clothes and holding his staff. He did not have any fear. Maybe he knew that he would be killed and martyred. However, it was no longer important. For, he became a member of the ummah of the Prophet of the end of time. He addressed the people as follows:
"O Greek congregation! We received a letter from Ahmad, the Prophet. He invites us to believe in Allah. I witness that there is no god but Allah and that Ahmad is His slave and Messenger."
The people attacked him all together like an arrow leaving the bow. And they did not leave that blessed man before martyring him. After that, Hz. Dihya went back to the emperor. He told him about the situation. The emperor said, "Did I not tell you that I fear them about our lives? I swear that Daghatir was more respectable than me in their eye." Then, he gave Dihya a lot of gifts and sent him.
On the way back to Madinah, Dihya was attacked by highwaymen. They grabbed everything that he had. Dihya entered Madinah empty-handed. He entered into the presence of the Messenger of Allah immediately. He told him everything that had happened. He gave the letter of the Greek emperor to him. The Prophet read the letter and said,
"He will rule his country for a while. Their sultanate will continue as long as my letter is with them."
The emperor also understood that the continuation of his sultanate depended on that letter. He wrapped the letter of the Messenger of Allah in atlas silk and placed it in a pipe made of gold. He willed that those who came after keep it. Indeed, it happened like that. When the letter disappeared, this dynasty lost its sultanate.
Hz. Dihya continued to be in the service of the Messenger of Allah after that. After the death of the Prophet, he served Allah and His Messenger during the era of the Four Caliphs. He participated in the Expedition of Syria during the caliphate of Hz. Abu Bakr. He took part in the Battle of Yarmuq during the caliphate of Hz. Umar. He fought heroically. He settled there after the conquest of Damascus. He died during the caliphate of Muawiya in 50 H.
The Emperor Heraclius wrote a letter to the Prophet (pbuh) after Dihya left. He sent a person who was reliable and who was known as "Tanuhi" to the Messenger of Allah (pbuh) and made him the following offer:
"Take this letter to that person. Listen to his words carefully. Memorize the words about three things that he will tell you. These are the three things: Will he mention the letter that he sent me? Will he remember the night when he reads my letter? Does he have anything that will make you suspicious on his back?"
Tanuhi took the letter of the emperor and set off. After a journey that took months, he found the Messenger of Allah in Tabuk. The Messenger of Allah was talking to his Companions at that time near some water. He went over to the Messenger of Allah and gave the letter of Heraclius to him; then, he sat down. Then, the following conversation took place between them:
“Who are you?”
“I am Tanuhi.”
“Are a member of the religion of Hanif, the religion of your ancestor Ibrahim?”
“I am an envoy of a nation and I belong to the religion of that nation. I will not abandon their religion unless I return to the nation that I represent.”
"Tanuhi brother! Allah guides whom He wishes to the right path. I wrote a letter to your emperor. He did not answer it."
Tanuhi realized that the Messenger of Allah had responded to one of the three things the emperor had asked him when he heard these words. He took an arrow out of his quiver and made a note on the sheath of his sword.
Then, the Messenger of Allah took the letter that came from the emperor from his breast, gave it to Muawiya, who was sitting on his left, and asked him to read it. The emperor wrote the following in his letter:
"You are calling me to a vast Paradise that was prepared for pious people and that covers the earth and the sky. Where is Hell? "
The Messenger of Allah replied:
"O my Allah! Where is night when it is daytime? "
With this statement, the Messenger of Allah mentioned the second issue that Heraclius asked. Tanuhi wrote this down immediately on the sheath of his sword.
After the reading of the letter was finished, the Messenger of Allah gave Tanuhi to Hz. Uthman, who would entertain him. While they were about to leave, the Messenger of Allah called: "O Tanuhi! Come here." The Messenger of Allah took off the garment on his back and showed him the seal of prophethood, which was between his two shoulder blades by saying, "Here is what you were asked about." Thus, the Prophet (pbuh) answered the three questions that the king had asked. Tanuhi went to the emperor and told him about what had happened.
Taking strength from what Tanuhi told him, Heraclius summoned the notables of Byzantium in his palace in Homs. He had all the doors shut and addressed them as follows:
"O notables of the Greek Land! Do you want to attain salvation, reach perfection and remain in the places that you have all the time? If so, come and be subject to this prophet."
The bigoted Greeks who heard these words began to growl and yell as if they were animals. They ran to the doors. In a sad and depressed manner, Heraclius ordered them to be taken to his presence again. He saw their hatred and gave up his hope that they might believe. The anxiety of protecting his sultanate and life prevented him from declaring his belief. He addressed the notables of the Greeks that gathered again as follows:
"I uttered the words that you heard a few minutes ago to see that whether you are adherent and loyal to your religion or not. I see that you are really adherent to your religion and be pleased with it."
Heraclius could not mention the Prophet and Islam again to the people around him. 
Tabaqat, 1: 258-259, 276. 4: 251; Musnad, 6: 262-263; Hz. Muhammed ve İslamiyet, 7: 38, 49.