Sahih Bukhari Volume 5, Book 58, Hadith Number 220.

Narated By Abu Huraira : That Allah’s Apostle informed them (i.e. his companions) of the death of Negus, the king of Ethiopia, on the very day on which the latter died, and said, “Ask Allah’s Forgiveness for your brother” Abu Huraira further said, “Allah’s Apostle made them (i.e. the Muslims) stand in rows at the Musalla (i.e. praying place) and led the funeral prayer for the Negus and said four Takbir.”