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. (IT)
English reference (IT): Book 16, Hadith 1560 (IT)
Arabic reference (IT): Book 16, Hadith 1517 (IT)