Musnad Ahmad 457:25

'Abd al-Razzaq narrated to us, he said: Sufyan narrated to us, from al-A'mash, from Ibrahim al-Taymi, from his father, from Abu Mas'ud al-Ansari, who said: While I was beating a slave of mine, I heard a voice from behind me: "Know, O Abu Mas'ud," [repeated] three times. I turned around, and it was the Messenger of Allah, peace and blessings of Allah be upon him. He said: "By Allah, Allah has more power over you than you have over him." He said: So I swore that I would never beat a slave again.