Sahih Muslim Book - 32, Hadith - 4609

حَدَّثَنِى يُوسُفُ بْنُ حَمَّادٍ الْمَعْنِىُّ حَدَّثَنَا عَبْدُ الأَعْلَى عَنْ سَعِيدٍ عَنْ قَتَادَةَ عَنْ أَنَسٍ أَنَّ نَبِىَّ اللَّهِ -صلى الله عليه وسلم- كَتَبَ إِلَى كِسْرَى وَإِلَى قَيْصَرَ وَإِلَى النَّجَاشِىِّ وَإِلَى كُلِّ جَبَّارٍ يَدْعُوهُمْ إِلَى اللَّهِ تَعَالَى وَلَيْسَ بِالنَّجَاشِىِّ الَّذِى صَلَّى عَلَيْهِ النَّبِىُّ -صلى الله عليه وسلم-.

It was narrated from Anas that the Prophet of Allah (s.a.w) wrote to Chosroes, Caesar, the Negus and to every tyrant, calling them to Allah. That was not the Negus for whom the Prophet (s.a.w) offered the funeral prayer.