Hadith 24:21
Sahih (Al-Albani)
'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
Original Reference: Al-Adab Al-Mufrad 439