Sayyiduna Abu Mas'ud al-Ansari (may Allah be pleased with him) said: I was beating a slave of mine when suddenly I heard a voice behind me: "Know, O Abu Mas'ud! Know, O Abu Mas'ud!" three times. I turned around, and it was the Messenger of Allah (peace and blessings be upon him). He (peace and blessings be upon him) said: "By Allah, Allah has far more power over you than you have over this (slave) in this state." So I swore that I would never again beat any slave of mine.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14755