The Messenger of Allah ﷺ said, "Whoever says to his brother, 'You disbeliever,' one of the two has incurred it."
حدثنا مؤمل حدثنا سفيان حدثنا عبد الله بن دينار سمعت
قال رسول الله ﷺ من قال لأخيه يا كافر فقد باء بها أحدهما
Its isnad is sahih. It is a repetition of 5259 and 5260, and an abridgment of 5824.