It is narrated from Sayyiduna Wahshi bin Harb (may Allah be pleased with him) that when the Negus (al-Najashi) died, the Messenger of Allah (peace and blessings be upon him) said: "Your brother the Negus has died; stand up and offer the funeral prayer for him." A man said: O Messenger of Allah! How can we offer the funeral prayer for him when he died in his state of disbelief? He (peace and blessings be upon him) said: "Do you not hear the saying of Allah, the Mighty and Majestic: 'And indeed, among the People of the Book are those who believe in Allah and in what was revealed to you and what was revealed to them' [Surah Al Imran: 199], to the end of the verse?"
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18581