(26) باب بَيَانِ حَالِ إِيمَانِ مَنْ قَالَ لأَخِيهِ الْمُسْلِمِ يَا كَافِرُ
It is reported on the authority of Ibn 'Umar that the Apostle (may peace and blessings be upon him) observed:
When a man calls his brother an unbeliever, it returns (at least) to one of them.
Reference: Sahih Muslim 60 a
In-book reference: Book 1, Hadith 121
USC-MSA web (English) reference(deprecated numbering scheme): Book 1, Hadith 116