Musannaf Ibn Abi Shayba 7:250

Ibn ‘Uyaynah narrated to us, from Sulayman al-Ahwal, from Abu Ma’bad, from Ibn ‘Abbas, who said: Whoever swears an oath regarding his possession (slave) that he will strike him, his expiation is to leave him (refrain from striking him), and he has the expiation (of an oath) sufficient for him.