Al-Adab Al-Mufrad, Book 24, Hadith 21 (In-Book) ⓘ
'Abdullah ibn 'Umar reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "If a man says to his brother, 'You are an unbeliever,' it is true for one or the other of them."
حَدَّثَنَا إِسْمَاعِيلُ، قَالَ: حَدَّثَنَا مَالِكٌ، عَنْ عَبْدِ اللهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللهِ صلى الله عليه وسلم قَالَ: أَيُّمَا رَجُلٌ قَالَ لأَخِيهِ: كَافِرٌ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا.
also referenced under: Hadith 439 (Reference) & Book 24, Hadith 439 (English)