Musannaf Ibn Abi Shayba 13:686

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Ibn Abi Najih, from ‘Ata’, Tawus, and Mujahid, they said: "The one who commits slander (al-qadhif), if he repents, his testimony is permissible."