The Messenger of Allah ﷺ said, "If any man says to his brother, 'You disbeliever,' the charge falls upon one of the two."
حدثنا إسحاق أخبرنا مالك عن عبد الله بن دينار
قال رسول الله ﷺ أيما رجل قال لأخيه يا كافر فقد باء بها أحدهما
Its isnad is sahih, and it is found in Muwatta 3:148. It is a repeat of 5914.