It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that when the news of the death of the Negus (al-Najashi) reached the Prophet (peace and blessings be upon him), he (peace and blessings be upon him) said: "Go out and offer the funeral prayer for a brother of yours whom you have never seen." So we went out; the Prophet (peace and blessings be upon him) stepped forward and we formed rows behind him (peace and blessings be upon him). Then he (peace and blessings be upon him) led the prayer and we prayed too. When we had finished, the hypocrites began to say: "Look at this man! He has come out to offer the funeral prayer over a Christian unbeliever whom he never even saw." Then Allah Almighty revealed this verse: "And indeed, among the People of the Book are those who believe in Allah and in what has been revealed to you and what has been revealed to them, humbling themselves before Allah; they do not sell the verses of Allah for a small price" to the end of the verse.
Hadith Referenceالمعجم الأوسط / باب العين / 4645
Hadith Takhrij«أخرجه الطبراني فى «الأوسط» برقم: 9409»