Musannaf Ibn Abi Shayba 19:150

Abu Bakr narrated to us, he said: Makhlad bin Yazid narrated to us, from Ibn Jurayj, from Umar bin Ata bin Abi al-Huwar, from Ikrimah, the freed slave of Ibn Abbas, regarding the slave who accuses a free person of adultery (qadhf): He said: He is to be flogged forty lashes.