Someone says calls his brother an unbeliever
'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