Hadith 517940
Chapter 56: Speech - كتاب الكلام
حَدَّثَنِي مَالِكٌ، عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ، عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ " مَنْ قَالَ لأَخِيهِ يَا كَافِرُ . فَقَدْ بَاءَ بِهَا أَحَدُهُمَا " .
Malik related to me from Abdullah ibn Dinar from Abdullah ibn Umar that the Messenger of Allah, may Allah bless him and grant him peace, said, "If a man says to his muslim brother, 'O kafir!' it is true about one of them."
Reference Muwatta Malik 517940
USC-MSA web (English) reference : Book 56, Hadith 1