(205) بَابُ مَنْ قَالَ لأَخِيهِ: يَا كَافِرُ
'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allahbless him and grant him peace, said, "If a man says to his brother, 'Youare an unbeliever,' it is true for one or the other of them."
Reference: Al-Adab Al-Mufrad 439
In-book reference: Book 24, Hadith 21
English translation: Book 24, Hadith 439