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