(205) بَابُ مَنْ قَالَ لأَخِيهِ: يَا كَافِرُ
'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allahbless him and grant him peace, said, "When someone says to another, 'Unbeliever!'then one of them is an unbeliever. If the one to whom he says it is anunbeliever, he has spoke the truth. If that is not the case, then the onewho said it has brought down disbelief on himself."
Reference: Al-Adab Al-Mufrad 440
In-book reference: Book 24, Hadith 22
English translation: Book 24, Hadith 440