Sayyiduna Ibn Umar (may Allah be pleased with them both) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "Who will take this letter of mine to the tyrant of Rome?" He (peace and blessings be upon him) put this to them three times, then said: "Who will go, and Paradise is his?" A man from the Ansar called Ubaydullah bin Abd al-Khaliq said: I will take it; and if I perish before that, will Paradise be mine? He (peace and blessings be upon him) said: "Yes. If you reach him, Paradise is yours; and if you are killed, and if you perish, Allah has made Paradise obligatory for you." So he set off with the letter of the Prophet (peace and blessings be upon him) until he reached the tyrant king, and he said: I am the envoy of the Messenger of the Lord of the worlds. He was given permission and went in to him. The tyrant of Rome recognised that he had come with the truth from a Prophet who had been sent. Then he laid the letter of the Prophet (peace and blessings be upon him) before him, and he gathered the Romans around him and laid it before them, but they disliked what he had brought. One man among them believed in it, and he was killed for his faith. Then the man returned to the Prophet (peace and blessings be upon him) and told him (peace and blessings be upon him) what had happened to him, and also about the killing of that man. At this the Prophet (peace and blessings be upon him) said: "Allah will raise him (on the Day of Resurrection) as a nation by himself", concerning that man who was killed.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13551