Sahih Muslim Book 27, Hadith 52
Shu'ba informed that Muhammad b. Munkadir said to me: What is your name? The rest of the hadith is the same.
Don't you know that it is forbidden (to strike the) face. He said: You see I was the seventh one amongst my brothers during the lifetime of Allah's Messenger (ﷺ), and we had but only one servant. One of us got enraged and slapped him. Thereupon Allah's Messenger (ﷺ) commanded us to set him free.
I was beating my slave with a whip when I heard a voice behind me: Understand, Abu Masud; but I did not recognise the voice due to intense anger. He (Abu Mas'ud) reported: As he came near me (I found) that he was the Messenger of Allah (ﷺ) and he was saying: Bear in mind, Abu Mas'ud; bear in mind. Abu Mas'ud. He (Aba Maslad) said: threw the whip from my hand. Thereupon he (the Holy Prophet) said: Bear in mind, Abu Mas'ud; verily Allah has more dominance upon you than you have upon your slave. I (then) said: I would never beat my servant in future.