1. The Book of Faith

١۔ كتاب الإيمان

al-Luʾluʾ wa-al-Marjān > Chapter 1 > Section 28
اللؤلؤ والمرجان > كتاب ١ > باب ٢٨

1.28 Clarifying the condition of faith for one who says to his muslim brother: "O Kafir (Disbeliever)."

١۔٢٨ باب بَيَانِ حَالِ إِيمَانِ مَنْ قَالَ لأَخِيهِ الْمُسْلِمِ يَا كَافِرُ

You would risk becoming a disbeliever if you call another Muslim a kāfir (disbeliever)

(39) bukhari:6104 – Ismāʿīl > Mālik > ʿAbdullāh b. Dīnār > ʿAbdullāh b. ʿUmar

The Messenger of Allah ﷺ said, "If anyone says to his brother O kāfir (disbeliever)! Then surely, one of them such."  

(٣٩) البخاري:٦١٠٤ – حَدَّثَنَا إِسْمَاعِيلُ قَالَ حَدَّثَنِي مَالِكٌ عَنْ عَبْدِ اللَّهِ بْنِ دِينَارٍ عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ؓ

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ: «أَيُّمَا رَجُلٍ قَالَ لأَخِيهِ: يَا كَافِرُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا۔»