Hadithcore
Mishkat al-Masabih, 3928

Anas told that the Prophet (ﷺ) wrote to Kisra, Qaisar, the Najashi, and every powerful man, summoning them to God, but it was not the Najashi on whom the Prophet (ﷺ) invoked a blessing.* * It says that the Najashi (Negus) of Abyssinia to whom this letter was sent was not the same as the one who ruled at the time when the early emigration from Mecca to Abyssinia took place. Muslim transmitted it.

وَعَنْ أَنَسٍ: أَنَّ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ كَتَبَ إِلَى كِسْرَى وَإِلَى قَيْصَرَ وَإِلَى النَّجَاشِيِّ وَإِلَى كُلِّ جَبَّارٍ يَدْعُوهُمْ إِلَى اللَّهِ وَلَيْسَ بِالنَّجَاشِيِّ الَّذِي صَلَّى عَلَيْهِ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ. رَوَاهُ مُسْلِمٌ

References1 variant
In-Book Reference
Book 19, Hadith 140