Allah's Messenger (ﷺ) said, "If a man says to his brother, O Kafir (disbeliever)!' Then surely one of them is such (i.e., a Kafir). "
Sahih al-Bukhari Book 78, Hadith 130
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Sahih al-Bukhari Book 78, Hadith 129 Next Hadith → Allah's Messenger (ﷺ) said, 'If anyone says to his brother, 'O misbeliever! Then surely, one of them such."

Sahih al-Bukhari Book 78, Hadith 131