Malik related to me from Abdullah ibn Dinar from Abdullah ibnUmar that the Messenger of Allah, may Allah bless him and grant himpeace, said, "If a man says to his muslim brother, 'O kafir!' it istrue about one of them." (IT)
USC-MSA web (English) reference: Book 56, Hadith 1 (IT)
Arabic reference (IT): Book 56, Hadith 1814 (IT)