Musannaf Ibn Abi Shayba 19:776

Abu Bakr narrated to us, he said: Ibn ‘Ulayyah narrated to us, from Yunus, from al-Hasan, he said: If a man divorces his wife three times, then accuses her of adultery (qadhf), he said: He is to be flogged with the prescribed punishment (hadd), unless she is pregnant. If she is pregnant, he performs li’an (imprecation) with her.