(73) باب مَنْ كَفَّرَ أَخَاهُ بِغَيْرِ تَأْوِيلٍ فَهْوَ كَمَا قَالَ
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 6104
In-book reference: Book 78, Hadith 131
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 8, Book 73, Hadith 125