The Prophet ﷺ said, "If a man says to his brother, 'You disbeliever,' then one of the two has acknowledged it."This is a hasan sahih gharib hadith. The meaning of his statement "baa" is that he has acknowledged it.
حدثنا قتيبة عن مالك عن عبد الله بن دينار
عن النبي ﷺ قال أيما رجل قال لأخيه كافر فقد باء به أحدهما هذا حديث حسن صحيح غريب ومعنى قوله باء يعني أقر
Agreed upon (by Bukhari and Muslim). Bukhari narrated it in the Book of Manners, Chapter on One Who Declares His Brother a Disbeliever Without Justification Then It Applies to Him As He Said, hadith no. 6104, from the hadith of Malik, and Muslim, hadith no. 60, from the hadith of Abdullah bin Dinar with the same wording. It is also found in Muwatta of Imam Malik, may Allah have mercy on him, 22/984, with a similar meaning.