Mu'jam Kabir Tabarani 4183 — The Prophet ﷺ sent me with a letter

Sayyiduna Dihyah al-Kalbi (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) sent me with a letter to Caesar, the ruler of the Romans. I said (to the gatekeeper): "Seek permission for the messenger of the Messenger of Allah (peace and blessings be upon him)." So someone went to Caesar and he was told: "There is a man at the door who claims that he is the messenger of the Messenger of Allah (peace and blessings be upon him)." They were alarmed at that, and he (the king) said: "Bring him in." So I was brought in to him, and his patricians (bataricah) were with him. I gave him the letter and it was read out to him, and in it was: "In the name of Allah, the Most Gracious, the Most Merciful. From Muhammad, the Messenger of Allah, to Caesar, the ruler of the Romans." At this a nephew of his, ruddy, blue-eyed and straight-haired, snorted and said: "Do not read the letter today. He has begun with himself, and he has written 'ruler of the Romans'; he did not write 'king of the Romans'." The narrator said: So the letter was read until they finished it. Then he ordered them, and they left his presence. Then he sent for me and I went in to him; he questioned me and I informed him. Then he sent for the bishop, who came to him; he was the one in charge of their affairs, and they acted upon his opinion and his word. When he read the letter, the bishop said: "By Allah, he is the one whose glad tidings Musa and Isa gave us, the one we have been waiting for." Caesar said: "Then what do you advise me?" He said: "As for me, I believe him and follow him." Caesar said: "I know that it is so, but I cannot do it. If I do it, my kingship will be gone and the Romans will kill me."