Abū Hurairah reports Allah’s Messenger (peace and blessings of Allah be to him) as saying: “The servant (who conceals) the faults of others in this world, Allah would conceal his faults on the Day of Resurrection.”
(Sahih Muslim)
Talking ill of a person is backbiting. It goes a long way in degrading a person. It is bound to result in ruining personal relationships. This hadīth teaches the believers to conceal the faults of others.
One should avoid publicising the faults and shortcomings of others, and thus save himself from the sin of degrading a brother. A Muslim is the brother of another Muslim and as such he deserves his help, cooperation and assistance. But in case there is genuine necessity, one should readily disclose them; and if one hesitates in doing so, one would be answerable for it on the Day of Resurrection. If a person has committed any cognisable offence and another person knows it, the latter should inform the authorities about it. If a ruler or a person in power commits evil deeds, it should be publicised, as concealment of these leads to ruin the society. If a man has to marry his daughter and he asks another person about the character of one with whom he intends to marry her, it is the duty of the person whose counsel has been sought for to give him correct information about the person.
It should, however, be kept in mind that the oppressed person is allowed to speak against the oppressor even in his absence.