Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.107B.
Section : Atonement for Backbiting.
It is related that the Prophet, peace be upon him, said, “The atonement for backbiting [and slander] is to pray for forgiveness of the person who was slandered and to say, ‘O Allah, forgive us and him’.”
The preferable course in this regard is to pray for forgiveness of the person who was the target of the slander and to mention his good qualities. This will wipe off the effects of such backbiting, and it is not necessary to announce such a thing.