Hadith: The Prophet of Allah (ﷺ) wrote to Khosrow, Caesar, the Negus, and every other tyrant, calling them to Allah Almighty

عَنْ أَنَسٍ رَضِيَ اللَّهُ عَنْهُ:
أَنَّ نَبِيَّ اللهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ كَتَبَ إِلَى كِسْرَى، وَإِلَى قَيْصَرَ، وَإِلَى النَّجَاشِيِّ، وَإِلَى كُلِّ جَبَّارٍ يَدْعُوهُمْ إِلَى اللهِ تَعَالَى، وَلَيْسَ بِالنَّجَاشِيِّ الَّذِي صَلَّى عَلَيْهِ النَّبِيُّ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ.
[صحيح] - [رواه مسلم] - [صحيح مسلم: 1774]

Anas (may Allah be pleased with him) reported:
The Prophet of Allah (ﷺ) wrote to Khosrow, Caesar, the Negus, and every other tyrant, calling them to Allah Almighty. And this Negus was not the same one over whom the Prophet (ﷺ) performed the funeral prayer.
[Authentic hadith] - [Narrated by Muslim] - [Sahih Muslim - 1774]

Explanation

Anas ibn Mālik (may Allah be pleased with him) informed that the Prophet (ﷺ), before his death, sent letters to the kings of the nations around him, inviting them to Islam. He wrote a letter to Khosrow (a title for every ruler of Persia), to Caesar (a title for every ruler of the Romans), and to the Negus (a title for the kings of Abyssinia). He also wrote to every tyrannical king who subjugated and oppressed people. Anas (may Allah be pleased with him) clarified that the Negus to whom the letter was sent was not the same Negus who had embraced Islam, passed away, and for whom the Prophet (ﷺ) performed the absentee funeral prayer.

Benefits from the Hadith

1. The legitimacy of inviting non-Muslims to Islam, including their kings and rulers.
2. The legitimacy of using written documents and letters.