Any person who says to his brother: O disbeliever, it will, indeed, fall upon one of them; if he is as he said, or else it will return to him

The Prophet (may Allah's peace and blessings be upon him) warned against a Muslim saying to his Muslim brother: O disbeliever. By saying that word of disbelief, one of them deserves it; if he is as he said, then it applies to the person being addressed, or else the charge of disbelief returns to the sayer.