I was beating a slave boy of mine when I heard a voice behind me saying, "Know, Abu Masud, that Allah has more power over you than you have over him."I turned around, and there was the Messenger of Allah ﷺ.I said, "Messenger of Allah ﷺ, he is free for the sake of Allah."He said, "Had you not done so, the Fire would have scorched you," or, "the Fire would have touched you."
كنت أضرب غلاما لي، فسمعت من خلفي صوتا: اعلم، أبا مسعود، الله أقدر عليك منك عليه، فالتفت فإذا هو رسول الله ﷺ، فقلت يا رسول الله ﷺ، هو حر لوجه الله، فقال: أما لو لم تفعل للفحتك النار، أو لمستك النار.
Muslim narrated it in the chapter on oaths and vows (1659:35), from Abu Kurayb Muhammad bin Ala, who said, Abu Muawiya told us, Amash told us, from Ibrahim Taymi, from his father, from Abu Masud Ansari, who said, and then he mentioned it.