The Messenger of Allah ﷺ said, "When a man says to his brother, 'You disbeliever,' one of the two has incurred it."
أن رسول الله ﷺ قال: إذا قال الرجل لأخيه: يا كافر فقد باء به أحدهما.
Bukhari reported it in Adab (6102), from Muhammad and Ahmad bin Saeed, who both said, "Uthman bin Umar narrated to us, Ali bin Mubarak informed us, from Yahya bin Abu Kathir, from Abu Salama, from Abu Hurairah," and he mentioned the hadith.