Good Manners and Form (Al-Adab)
78/73. Chapter:
Whoever calls his brother a Kafir without any grounds
Sahih Bukhari
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 ".
Narrated 'Abdullah bin 'Umar: Allah's Messenger (ﷺ) said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such."
Reference : Sahih al-Bukhari 6104In-book reference : Book 78, Hadith 131USC-MSA web (English) reference : Vol. 8, Book 73, Hadith 125