The Messenger of Allah ﷺ said, "Whoever says to his brother, 'You disbeliever,' one of them has incurred it."
أن رسول الله ﷺ قال: من قال لأخيه: يا كافر، فقد باء بها أحدهما.
Malik narrated it in speech (1) from Abdullah bin Dinar, from Abdullah bin Umar, who said, then he mentioned it.Bukhari narrated it in manners (6104) through Malik, with the same wording.Muslim narrated it in faith (60) through Ismail bin Jafar, from Abdullah bin Dinar, with the same chain, and he added in it, "If it is as he said, otherwise it returns upon him."