Full Sharh — Mishkat al-Masabih 3765
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
English Translation
Anas told that the Prophet wrote to Kisra, Qaisar, the Najashi, and every powerful man, summoning them to God, but it was not the Najashi on whom the Prophet 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.
Source
Mishkat al-Masabih(مشكاة المصابيح)
Jihad
Hadith Number
3765