Sayyiduna Abu Mas'ud (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!" I turned around, and it was the Messenger of Allah (peace and blessings be upon him). Then the narrator mentioned a similar hadith.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14758