Musnad Abu Ya'la al-Mawsili 100:1

Hawthara ibn Ashras narrated to us, Hammad ibn Salama narrated to us, from Abdullah ibn Uthman ibn Khuthaym, from Sa’id ibn Abi Rashid, who said: The messenger of Caesar was a neighbor of mine during the time of Yazid ibn Mu’awiyah. I said to him, "Inform me about the letter of the Messenger of Allah (peace and blessings of Allah be upon him) to Caesar." He said: "The Messenger of Allah (peace and blessings of Allah be upon him) sent Dihya al-Kalbi to Caesar and wrote a letter with him to him, giving him a choice between one of three things: either he accepts Islam and he shall have what is in his hands of his kingdom, or he pays the tribute (kharaj), or he permits war." He said: "Caesar gathered his patriarchs and priests in his palace and locked the door upon them. He said, 'Muhammad has written to me, giving me a choice between one of three things: either I accept Islam and I shall have what is in my hands of my kingdom, or I pay the tribute, or I permit war. And you find in what you read of your books that he will surely possess what is beneath my feet of my kingdom.' They snorted a snort, to the extent that some of them came out of their cloaks and said, 'Do you submit to a man from the Arabs who came in his two garments and his two sandals, regarding the tribute?' He said, 'Be silent. I only wanted to know your adherence to your religion and your desire for it.'" "Then he said, 'Seek for me a man from the Arabs.' So they brought me. He wrote with me a letter to the Prophet (peace and blessings of Allah be upon him) and said to me, 'Observe what falls from you of his words, so that it does not fall when the night and the day are mentioned.' I came to the Messenger of Allah (peace and blessings of Allah be upon him) while he was with his companions, and they were sitting with their knees drawn up to their chests, with the straps of their swords around them, around the well of Tabuk. I said, 'Which of you is Muhammad?' He gestured with his hand toward himself. I handed the letter to him, and he handed it to a man beside him. I said, 'Who is this?' They said, 'Mu’awiyah ibn Abi Sufyan.' He read it, and behold, in it was written: 'You invite me to a Paradise whose width is the heavens and the earth, so where is the Fire then?' The Messenger of Allah (peace and blessings of Allah be upon him) said, 'Glory be to Allah! When the night comes, where is the day?' I wrote it down with me." "Then the Messenger of Allah (peace and blessings of Allah be upon him) said, 'You are the messenger of a people, so you have a right, but you have come to us while we are destitute.' Uthman said, 'Clothe him in a yellow garment.' A man from the Ansar said, 'His hospitality is upon me.' Caesar had said to me, among what he said, 'Look at his back.' The Messenger of Allah (peace and blessings of Allah be upon him) perceived that I wanted to look at his back, so he cast his garment off his back. I looked at the Seal in the depression of the shoulder, and I approached it and kissed it. Then the Messenger of Allah (peace and blessings of Allah be upon him) said, 'I wrote to the Negus, and he burned my letter; by Allah, He will burn him. And I wrote to Chosroes, the Great of Persia, and he tore up my letter; by Allah, He will tear him up. And I wrote to Caesar, and he raised my letter; so the people will continue [to have]...'" (The narrator mentioned a word regarding there being good in life).