(27) باب كُتُبِ النَّبِيِّ صلى الله عليه وسلم إِلَى مُلُوكِ الْكُفَّارِ يَدْعُوهُمْ إِلَى اللَّهِ عَزَّ وَجَلَّ
It has been narrated on the authority of Anas that the Prophet of Allah (ﷺ) 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 (ﷺ) had said the funeral prayers.
Reference: Sahih Muslim 1774 a
In-book reference: Book 32, Hadith 91
USC-MSA web (English) reference(deprecated numbering scheme): Book 19, Hadith 4382