Al-Amālī (al-Mufīd), no. 195

7. He said: Abu Ubaidullah Muhammad ibn Imran al-Marzbani reported to me from Muhammad ibn Ahmad al-Hakimi, who reported from Muhammad ibn Ishaq, who reported from Dawood ibn al-Muhabbar, who reported from Ambasah ibn Abdul Rahman al-Qarshi, who reported from Khalid ibn Yazid al-Yamani, who reported from Anas ibn Malik that: The Prophet, peace be upon him and his progeny, said: "The penance and expiation for backbiting anyone is to seek forgiveness from Allah for him or her." And may Allah bless His mercy upon our master Muhammad and his progeny.