Mishkat al-Masabih 13 Marriage كتاب النكاح 265

وَعَنْ أَبِي هُرَيْرَةَ قَالَ: سَمِعْتُ أَبَا الْقَاسِمِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ: «مَنْ قَذَفَ مَمْلُوكَهُ وَهُوَ بَرِيءٌ مِمَّا قَالَ جُلِدَ يَوْمَ الْقِيَامَةِ إِلَّا أَنْ يَكُونَ كَمَا قَالَ»

Abu Huraira told that he heard Abul Qasim (i.e. the Prophet) say, “If any one reviles his slave when he is innocent of what he said, he will be beaten on the day of resurrection, unless it is as he said.” (Bukhari and Muslim.)