Sahih Muslim | Book 19 | Hadith Number 4382

Narrated by Anas
It has been narrated on the authority of Anas that the Prophet of Allah (may peace be upon him) wrote to Chosroes (King of Persia), Caesar (Emperor of Rome), Negus (King of Abyssinia) and every (other) despot inviting them to Allah, the Exalted. And this Negus was not the one for whom the Messenger of Allah (may peace be upon him) had said the funeral prayers.