Musnad Ahmad 27:628

He said: I found in my father’s book: Hajjaj narrated to us, Shu’bah informed us, from ‘Abdullah ibn Dinar, from Ibn ‘Umar, from the Prophet, may Allah bless him and grant him peace, that he said: "If a man says to his brother, 'You are a disbeliever' or 'O disbeliever,' then one of the two has incurred it."