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.
It is reported on the authority of Ibn 'Umar that the Messenger of Allah (may peace and blessings be upon him) said
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).