Shaykh al-Islam Ibn Taymiyah said:
“Whoever wrongs a person by slandering him, backbiting about him or insulting him, then repents, Allah will accept his repentance, but if the one who was wronged finds out about that, he has the right to settle the score. But if he slandered him or backbit about him and the person did not hear of that, then there are two views according to the scholars, both of which were narrated from Ahmad, the more correct of which is that he should not tell him that he spoke against him in his absence. It was said that he should rather speak well of him in his absence just as he spoke badly of him in his absence, as al-Hasan al-Basri said: the expiation for gheebah is to pray for forgiveness for the person about whom you backbit.”
Allah the All Mighty said in Quran: “O you who believe! Avoid much suspicions, indeed 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 Allah. Verily, Allah is the One Who accepts repentance, Most Merciful”. [049:012]
“No Laghw (dirty, false, evil vain talk) will they hear therein, nor any sinful speech (like backbiting, etc).” [056:025]
It was narrated that Ibn ‘Abbaas (R.A) said: The Messenger of Allah (pbuh) passed by two graves and said, “They are being punished, but they are not being punished for anything that was difficult to avoid. One of them used to walk about spreading malicious gossip (nameemah), and the other used not to take precautions to avoid getting urine on himself when he urinated.” Then he called for a green branch, which he split in two and planted a piece on each grave, and said, “May their torment be reduced so long as these do not dry out.” Narrated by al-Bukhaari, 213; Muslim, 292
Narrated from Abu Hurayrah (R.A) that the Messenger of Allaah (pbuh) said: “Do you know what gheebah (backbiting) is?” They said, “Allah and His Messenger know best.” He said, “Saying something about your brother that he dislikes.” It was said, “What if what I say about my brother is true?” He said, “If what you say is true then you have backbitten about him, and if it is not true, then you have slandered him.” Narrated by Muslim, 2589
It was narrated that Abu Hurayrah (R.A) said: The Messenger of Allaah (pbuh) said: “Whoever has wronged his brother with regard to his honour or anything else, let him seek his forgiveness today, before there will be no dinar and no dirham, and if he has any good deeds to his credit they will be taken from him in a manner commensurate with the wrong he did, and if he has no good deeds, then some of his counterpart’s bad deeds will be taken and added to his burden.” Narrated by al-Bukhaari, 2317.
Allah knows the best.