Musnad Abu Ya'la al-Mawsili 42:97

Dāwūd ibn Rashīd narrated to us, Ismāʿīl narrated to us, from Bard ibn Sinān, from Abū Hārūn, from Abū Saʿīd, that the Prophet of Allah (may Allah bless him and grant him peace) said: "If one of you strikes his servant and he mentions Allah, then withdraw your hands."