Backbiting and Slandering are Major Sins

“Then do not obey the deniers (8). They wish that you would soften [in your position], so they would soften [toward you] (9). And do not obey every worthless habitual swearer (10). [And] scorner, going about with malicious gossip (11).  A preventer of good, transgressing and sinful.” (Surah Al-Qalam, 68:8-12)

“(back biting) Woe to every (kind of) scandal-monger and backbiter.” (Surah Al-Humazah, 104:1)

“O ye who believe! avoid suspicion as much (as possible): for suspicion in some cases is a sin: and spy not on each other nor speak ill of each other behind their backs. Would any of you like to eat the flesh of his dead brother? Nay ye would abhor it... but fear Allah: for Allah is Oft-Returning Most Merciful.” (Surah Al-Hujurat, 49:1 2)

Once the Prophet (Peace be upon him) went through the graveyards of Medina and heard the voices of two humans who were being tortured in their graves. The Prophet (Peace be upon him) said, "They are being punished, but they are not being punished because of a major sin, yet their sins are great. One of them used not to save himself from (being soiled with) the urine, and the other used to go about with calumnies backbiting (Namima)." Then the Prophet (Peace be upon him) asked for a green palm tree leaf and split it into two pieces and placed one piece on each grave, saying, "I hope that their punishment may be decreased as long as these pieces of the leaf are not dried." (Reported by Bukhari 8:81)