1. It is impermissible to engage in gheebah (backbiting). Gheebah is to discuss some aspect of a person in his absence which he would not like people to speak about (e.g. the sins which he commits in secret). However, if a person is a flagrant sinner and commits sins in public, then if such a person’s sins are mentioned in order to caution people, it will not enter under the ambit of gheebah.
وَلَا يَغْتَب بَّعْضُكُم بَعْضًا ۚ أَيُحِبُّ أَحَدُكُمْ أَن يَأْكُلَ لَحْمَ أَخِيهِ مَيْتًا فَكَرِهْتُمُوهُ (حجرات:12)
And do not engage in backbiting among yourselves. Would any of you like to eat the flesh of his dead brother? You would abhor it.
عن أبي هريرة، أن رسول الله صلى الله عليه وسلم، قال: أتدرون ما الغيبة؟ قالوا: الله ورسوله أعلم، قال: ذكرك أخاك بما يكره قيل أفرأيت إن كان في أخي ما أقول؟ قال: إن كان فيه ما تقول، فقد اغتبته، وإن لم يكن فيه فقد بهته (صحيح مسلم، الرقم: 2589)
Hazrat Abu Hurairah (radhiyallahu ‘anhu) reports that Rasulullah (sallallahu ‘alaihi wasallam) asked, “Do you know what is gheebah?” The Sahaabah (radhiyallahu ‘anhum) replied, “Allah Ta‘ala and His messenger know best.” Rasulullah (sallallahu ‘alaihi wasallam) said, “Speaking about your brother (in his absence) of that which he would dislike.” A Sahaabi asked, “What if what I am saying is (true and) found in my brother (then too will it be gheebah)?” Rasulullah (sallallahu ‘alaihi wasallam) replied, “If what you say regarding him is (true and) found in him, then you have backbit him, and if what you mention is not found in him, then you have slandered him.”