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

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.

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

صحيح مسلم (3/ 1397) (1774). المفهم لما أشكل من تلخيص كتاب مسلم (3/ 612). البحر المحيط الثجاج في شرح صحيح الإمام مسلم بن الحجاج (31/ 67). النهاية في غريب الحديث والأثر (136).