question #3 Sahih Muslim 60b, Book 1, Hadith 122 : What is the significance of verifying accusations before making them in Islam?

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).