Sahih al-Bukhari 6104
Sahih al-Bukhari · Book 78: Good Manners and Form (Al-Adab) · Whoever calls his brother a Kafir without any grounds
Narrated 'Abdullah bin 'Umar:
حَدَّثَنَا إِسْمَاعِيلُ ، قَالَ حَدَّثَنِي مَالِكٌ ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ـ رضى الله عنهما ـ أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ يَا كَافِرُ. فَقَدْ بَاءَ بِهَا أَحَدُهُمَا " .
Allah's Messenger (ﷺ) said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such."