Musannaf Ibn Abi Shayba 13:694

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Isma’il bin Abi Khalid, from Abu al-Duha, from Shurayh, he said: The testimony of the one who commits qadhf (false accusation of unchastity) is not permissible, and his repentance is between him and Allah.