When a man calls his brother an unbeliever, it returns (at least) to one of them.
Sahih Muslim Book 1, Hadith 121
← Previous Hadith even if he observed fast and prayed and asserted that he was a Muslim.

Sahih Muslim Book 1, Hadith 120 Next Hadith → Any person who called his brother: "O unbeliever" (has in fact done an act by which this unbelief) would return to one of them. If it were so, as he asserted (then the unbelief of man was confirmed but if it was not true), then it returned to him (to the man who labeled it on his brother Muslim).

Sahih Muslim Book 1, Hadith 122