Al-Matalib al-'Aliyah 4316 — The Battle of Tabuk

It is narrated from Bakr bin Abdullah al-Muzani (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Who will take this letter of mine to Caesar (of Rome), and in return Paradise (is promised) for him?" A man said: (Will this reward be given) even if he is not killed? He (peace and blessings be upon him) said: "Yes, even if he is not killed." So that man set off, reached Caesar and presented the letter to him. He read the letter and said: Go back to your Prophet and tell him that I am his follower (a Muslim), but I do not wish to give up my kingdom. And he sent some dinars with the messenger to the Messenger of Allah (peace and blessings be upon him). The man returned and informed him (peace and blessings be upon him) of what had happened, and the Messenger of Allah (peace and blessings be upon him) said: "He has lied (in claiming to be my follower)." And he (peace and blessings be upon him) distributed those dinars.
This report is in Musnad al-Harith (4316). Bakr bin Abdullah al-Muzani is a Successor (Tabi'i), so this hadith is mursal (with a missing link).