Th worst of all sins are those that are greatest in harm and danger to humanity. Among the destructive major sins are backbiting and slandering.
These two sins are forbidden by Allah because they sow enmity, evils and discord among people and lead to destruction. They cause hostilities between people of the same household and between neighbors and relatives. They can decrease in good deeds and increase in evil ones and lead to dishonor and ignominy.
Backbiting (gheebah) means mentioning something about a person (in his absence), that he hates (to have mentioned), whether it is about: His body, his religious characteristics, his worldly affairs, his self, his physical appearance, his character, his wealth, his child, his father, his wife, his manner of walking, his smile, it is the same whether you mention that about him with words, through writings, or whether you point or indicate him by gesturing with your eyes, hand or head.
As for the body, is when you make fun of how someone looks, or mentioning any bad quality in him, as saying: "he is blind", "he limps", "he is bleary-eyed", "he is bald", "he is short", "he is tall", "he is black", "he is yellow", "he's too thin", "he's too fat". As for his religious qualities, it is when you say: "he is a sinner", "he is a thief", "he is a betrayer", "he is an oppressor", "he doesn't pray", "he prays so fast", "he does not behave well towards his parents", "he does not pay the Zakat duly"." As for the worldly matters, then it is when you say: "he has poor manners", "he does not think that anyone has a right over him", "he talks too much" …etc
O you who believe! Avoid much suspicion, in deeds some suspicions are sins. And spy not neither backbite one another. Would one of you like to eat the flesh of his dead brother? You would hate it (so hate backbiting). And fear God, verily, God is the one who accepts repentance, Most Merciful. (49: 12)
Prophet Muhammad said :"Do you know what backbiting is?" They said, "God and His Messenger know best." He then said, "It is to say something about your brother that he would dislike." Someone asked him, "But what if what I say is true?" The Messenger of God said, "If what you say about him is true, you are backbiting him, but if it is not true then you have slandered him." (Muslim)