Compiled by Imam Yahya ibn Sharaf al-Nawawi
عن ابن عمر رضي الله عنهما قال: قال رسول الله صلى الله عليه وسلم : "إذا قال رجل لأخيه: يا كافر، فقد باء بها أحدهما، فإن كان كما قال وإلا رجعت عليه" ((متفق عليه)).
The Messenger of Allah (ﷺ) said, "When a person calls his brother (in Islam) a disbeliever, one of them will certainly deserve the title. If the addressee is so as he has asserted, the disbelief of the man is confirmed, but if it is untrue, then it will revert to him."[Al-Bukhari and Muslim].