Musnad Ahmad 980:16

'Abd al-Rahman narrated to us, from Sufyan, from al-A'mash, from Ibrahim al-Taymi, from his father, from Abu Mas'ud, who said: While I was beating a slave of mine, a man called out from behind me: "Know, Abu Mas'ud! Know, Abu Mas'ud!" I turned around, and it was the Messenger of Allah (peace and blessings of Allah be upon him). He said: "By Allah, Allah is more capable of you than you are of him." He said: So I swore that I would never beat a slave of mine ever again.