Shu'ba said, "Muhammad ibn al-Munkadir said to me, 'What is your name?' I replied, 'Shu'ba.' He said, 'Abu Shu'ba related to me that when Suwayd ibn Muqarrin al-Muzani saw a man strike his slave, he said. 'Do you not know that the face is forbidden? In the time of the Messenger of Allah, may Allah bless him and grant him peace, we were seven brothers and we only had one servant. Then one of us slapped him (sic.) and the Prophet, may Allah bless him and grant him peace, commanded that we set him free.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا عَمْرُو بْنُ مَرْزُوقٍ، قَالَ: أَخْبَرَنَا شُعْبَةُ قَالَ لِي مُحَمَّدُ بْنُ الْمُنْكَدِرِ: مَا اسْمُكَ؟ فَقُلْتُ: شُعْبَةُ قَالَ: حَدَّثَنِي أَبُو شُعْبَةَ، عَنْ سُوَيْدِ بْنِ مُقَرِّنٍ الْمُزَنِيِّ، وَرَأَى رَجُلاً لَطَمَ غُلاَمَهُ، فَقَالَ: أَمَا عَلِمْتَ أَنَّ الصُّورَةَ مُحَرَّمَةٌ؟ رَأَيْتُنِي وَإِنِّي سَابِعُ سَبْعَةِ إِخْوَةٍ، عَلَى عَهْدِ رَسُولِ اللهِ صلى الله عليه وسلم، مَا لَنَا إِلاَّ خَادِمٌ، فَلَطَمَهُ أَحَدُنَا، فَأَمَرَنَا النَّبِيُّ صلى الله عليه وسلم أَنْ نُعْتِقَهُ.
sunnah.com