The Messenger of Allah ﷺ said, "If any man says to his brother, 'You disbeliever,' then one of them has incurred it."
أن رسول الله ﷺ قال: أيما رجل قال لأخيه: يا كافر، فقد باء بها أحدهما.
Malik narrated it in the chapter on speech, backbiting, and piety (1) from Abdullah bin Dinar, from Abdullah bin Umar, as mentioned. Through his route, Bukhari narrated it in the chapter on manners (6104). Muslim narrated it in the chapter on faith (60) through another route from Abdullah bin Dinar with his chain, and its wording is, "Whenever a man says to his brother, 'You are a disbeliever,' one of the two has earned that description. If it is as he said, then so be it, otherwise it falls back upon him."Likewise, it is not fitting for anyone to say to a Muslim whose father died as a disbeliever and who was harsh toward Islam and the Muslims, "This is the son of the enemy of Allah," as is found in the hadith that follows.