The Prophet ﷺ said, "When one of you says to his brother, 'You disbeliever,' one of the two has incurred it."
حدثنا يحيى عن سفيان حدثني ابن دينار سمعت
عن النبي ﷺ قال إذا أحدكم قال لأخيه يا كافر فقد باء بها أحدهما
Its isnad is sahih. Munziri said in al-Targhib wal-Tarhib 2/84, Malik, Bukhari, Muslim, Abu Dawud, and Tirmidhi narrated it."Baa bihi ahaduhuma" means he committed to it and returned bearing it, and the root meaning of "bawaa" is commitment, as stated by Ibn Athir.