Anas (RAA) narrated that the Messenger of Allah (ﷺ) said:
"The atonement of backbiting a man is to ask Allah to forgive him." Related by Al-Harith bin Abu Usamah with a weak chain of narrators. (FR)
English reference (FR): Book 16, Hadith 1560 (FR)
Arabic reference (FR): Book 16, Hadith 1517 (FR)