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).'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُحَمَّدُ بْنُ سَلاَمٍ، قَالَ: أَخْبَرَنَا أَبُو مُعَاوِيَةَ، عَنِ الأَعْمَشِ، عَنْ إِبْرَاهِيمَ التَّيْمِيِّ، عَنْ أَبِيهِ، عَنْ أَبِي مَسْعُودٍ قَالَ: كُنْتُ أَضْرِبُ غُلاَمًا لِي، فَسَمِعْتُ مِنْ خَلْفِي صَوْتًا: اعْلَمْ أَبَا مَسْعُودٍ، لَلَّهُ أَقْدَرُ عَلَيْكَ مِنْكَ عَلَيْهِ، فَالْتَفَتُّ فَإِذَا هُوَ رَسُولُ اللهِ صلى الله عليه وسلم، قُلْتُ: يَا رَسُولَ اللهِ، فَهُوَ حُرٌّ لِوَجْهِ اللهِ، فَقَالَ: أَمَا لَوْ لَمْ تَفْعَلْ لَمَسَّتْكَ النَّارُ أَوْ لَلَفَحَتْكَ النَّارُ.
sunnah.com