1. It is not permissible for a person to refer to someone in the following words: “May Allah’s punishment, curse, or anger descend on him”, “may he enter hell”, etc. If the person who has been addressed in this manner does not deserve such remarks, all these curses will actually descend and fall on the person who uttered them. Similarly, it is not permissible to call a believer a kaafir (disbeliever).
وعن أبي ذر قال: قال رسول الله صلى الله عليه وسلم: لا يرمي رجل رجلا بالفسوق ولا يرميه بالكفر إلا ارتدت عليه إن لم يكن صاحبه كذلك (صحيح البخاري، الرقم: 6045)
Hazrat Abu Zar (radhiyallahu ‘anhu) reports that Rasulullah (sallallahu ‘alaihi wasallam) said, “No person accuses another of transgression (and evil), nor of disbelief, except that it will return to him if that person is not worthy of it.”