Back biting (ghiba) is a major sin (gunah e kabira). In Qur’an it is described as eating the flesh of ones’ dead brother.
It is mentioning in front of others something (true) about an absent fellow Muslim that would be unpleasant to him if he were to be aware of it.
It is from the category of rights of the fellow Muslims (huquq al ibad). Repentance from it requires asking forgiveness from the aggrieved individual.
It was a habit of Hakim al-Umma Mawlana Ashraf Ali Thanawi (Allah have mercy on him) to ask the individual who approached him to apologize for back biting him,
‘I forgive you but, please, let me know what did you say about me.’
To his disciples he explained that I do so to find out that if he had criticized me genuinely. If it was the case then I will make effort to amend those mistakes.
Hakim al-Umma had a permanent feature in the monthly publication of his Khanqah, al Noor, regarding this issue. It was to acknowledge his errors and slips. He even hired scholars to vet his works.
One who is seeking Allah’s pleasure and acceptance is never hesitant to acknowledge his own mistakes and repent. Each and every criticism, however acrid it maybe provides an opportunity to do so. It is a blessing in disguise.
wa ma tawfiqi illa billah