Hadith: The Prophet (may Allah's peace and blessings be upon him) announced the death of the Negus on the day he had died. He took the people to the prayer place, lined them up, and made four Takbīrs

عن أبي هريرة رضي الله عنه قال: «نَعَى النبيُّ صلى الله عليه وسلم النَّجَاشِيَّ في اليوم الذي مات فيه، خرج بهم إلى المصلَّى، فصفَّ بهم، وكَبَّرَ أَرْبَعاً».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported: The Prophet (may Allah's peace and blessings be upon him) announced the death of the Negus on the day he had died. He took the people to the prayer place, lined them up, and made four Takbīrs.
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The Negus, king of Abyssinia, did the Muslim emigrants a generous favor. When Quraysh in Makkah oppressed Muslims, they sought refuge in Abyssinia before there was a Muslim community in Madīnah. There in Abyssinia, the Negus received them well and treated them with hospitality. His sincere intention and lack of arrogance led him to accept the truth and convert to Islam. He died in his country without seeing the Prophet (may Allah's peace and blessings be upon him). Because of his favors to Muslims, his high esteem, and dying in a land where no Muslim prayers were said over him, the Prophet (may Allah's peace and blessings be upon him) informed his Companions of his death and took them out to the prayer place to say prayers on him. By this, the Prophet (may Allah's peace and blessings be upon him) declared that the Negus had converted to Islam and also paid him tribute for his position and generosity. The Prophet's prayer with the Muslim congregation would be an intercession with Allah, the Almighty, in favor of the Negus.