Al-Adab Al-Mufrad #171

Abu Mas'ud said, "I was beating a slave of mine when I heard a voice behind me, 'Know, Abu Mas'ud, that Allah is able to call you to account for this slave.' I turned around and there was the Messenger of Allah, may Allah bless him and grant him peace. I said, 'Messenger of Allah, he is free for the sake of Allah!' He said, 'If you had not done that, the Fire would have touched you (or the Fire would have burned you).'"