كتاب الجهاد والسير
32  
The Book of Jihad and Expeditions
(27)
Chapter: The Prophet (saws) wrote to the Kings of the Kuffar, inviting them to Islam
(27)
باب كُتُبِ النَّبِيِّ صلى الله عليه وسلم إِلَى مُلُوكِ الْكُفَّارِ يَدْعُوهُمْ إِلَى اللَّهِ عَزَّ وَجَلَّ
Sahih Muslim 1774 a

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.

حَدَّثَنِي يُوسُفُ بْنُ حَمَّادٍ الْمَعْنِيُّ، حَدَّثَنَا عَبْدُ الأَعْلَى، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ نَبِيَّ اللَّهِ صلى الله عليه وسلم كَتَبَ إِلَى كِسْرَى وَإِلَى قَيْصَرَ وَإِلَى النَّجَاشِيِّ وَإِلَى كُلِّ جَبَّارٍ يَدْعُوهُمْ إِلَى اللَّهِ تَعَالَى وَلَيْسَ بِالنَّجَاشِيِّ الَّذِي صَلَّى عَلَيْهِ النَّبِيُّ صلى الله عليه وسلم ‏.‏
Reference : Sahih Muslim 1774a
In-book reference : Book 32, Hadith 91
USC-MSA web (English) reference : Book 19, Hadith 4382
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