Musnad Ahmad 136:1

Ishaq bin Isa narrated to us, he said: Yahya bin Sulaym narrated to me, from Abdullah bin Uthman bin Khuthaym, from Sa’id bin Abi Rashid, who said: I met the Tanukhi, the messenger of Heraclius to the Messenger of Allah (peace and blessings of Allah be upon him), in Homs. He was a neighbor of mine, an old man who had reached the age of senility or was close to it. I said: "Will you not tell me about the letter of Heraclius to the Prophet (peace and blessings of Allah be upon him), and the letter of the Messenger of Allah (peace and blessings of Allah be upon him) to Heraclius?" He said: "Yes. The Messenger of Allah (peace and blessings of Allah be upon him) arrived at Tabuk, and he sent Dihyah al-Kalbi to Heraclius. When the letter of the Messenger of Allah (peace and blessings of Allah be upon him) reached him, he summoned the priests and patriarchs of Rome, then locked a door upon himself and them. He said: 'This man has descended where you have seen, and he has sent to me inviting me to three things: he invites me to follow him in his religion, or that we give him our wealth in our land while the land remains ours, or that we wage war against him. By Allah, you know from what you read in the scriptures that he will surely take what is beneath my feet. So come, let us follow him in his religion, or give him our wealth in our land.' They snorted as one man, until they emerged from their robes and said: 'Do you invite us to abandon Christianity, or to become slaves to a Bedouin who came from the Hijaz?'" "When he thought that if they left him, they would incite the Romans against him, he appeased them—though he barely did—and said: 'I only said that to you to test your steadfastness in your affair.' Then he summoned a man from the Arab tribe of Tujib who was over the Arab Christians and said: 'Summon for me a man who is a memorizer of speech, of Arab tongue, that I may send him to this man with the reply to his letter.' So I was brought to him. Heraclius handed me a letter and said: 'Go with my letter to this man, and do not lose his speech. Memorize for me three things from him: observe if he mentions his scroll which he wrote to me regarding anything; observe when he reads my letter if he mentions the night; and observe his back, is there anything upon it that makes you suspicious?'" "I set out with his letter until I came to Tabuk. He was sitting among his companions, sitting with his knees drawn up to his chest (muhtabi) by the water. I said: 'Where is your companion?' It was said: 'There he is.' I approached walking until I sat before him. I handed him my letter, and he placed it in his lap. Then he said: 'Who are you from?' I said: 'I am one of the Tanukh.' He said: 'Do you have any interest in Islam, the upright religion of your father Ibrahim?' I said: 'I am a messenger of a people, and I am upon the religion of a people; I will not turn back from it until I return to them.' He laughed and said: *'Indeed, you do not guide whom you like, but Allah guides whom He wills. And He is most knowing of the [rightly] guided.'* (28:56). 'O brother of Tanukh, I wrote a letter to Chosroes and he tore it up; by Allah, He will tear him up and tear up his kingdom. I wrote to the Negus with a scroll, and he tore it up; by Allah, He will tear him up and tear up his kingdom. I wrote to your companion with a scroll, and he held onto it; people will continue to find strength from him as long as there is good in life.'" "I said: 'This is one of the three things my companion charged me with.' I took an arrow from my quiver and wrote it on the leather of my sword. Then he handed the scroll to a man on his left. I said: 'Who is the companion of your letter who reads for you?' They said: 'Muawiyah.' It was in my companion's letter: 'You invite me to a Paradise whose width is the heavens and the earth, prepared for the righteous; so where is the Fire?' The Messenger of Allah (peace and blessings of Allah be upon him) said: 'Subhan Allah! Where is the night when the day comes?' I said: 'I took an arrow from my quiver and wrote it on the leather of my sword.'" "When he finished reading my letter, he said: 'You have a right, and you are a messenger. If we had a reward here, we would reward you with it, but we are travelers who are destitute.' A man from the group of people called out: 'I will reward him.' He opened his saddlebag and brought out a Safuri garment and placed it in my lap. I said: 'Who is the owner of the reward?' It was said to me: 'Uthman.' Then the Messenger of Allah (peace and blessings of Allah be upon him) said: 'Who among you will host this man?' A young man from the Ansar said: 'I will.' The Ansari stood up, and I stood with him. When I had left the side of the gathering, the Messenger of Allah (peace and blessings of Allah be upon him) called me and said: 'Come, O brother of Tanukh.' I approached, moving toward him, until I was standing in the place where I had been before him. He loosened his garment from his back and said: 'Here, proceed with what you were ordered to do.' I looked at his back, and there I was, seeing a seal in the place of the folds of the shoulder, like a large cupping mark."