I heard Abu-l-Qasim (the Prophet) saying, "If somebody slanders his slave and the slave is free from what he says, he will be flogged on the Day of Resurrection unless the slave is really as he has described him."  

سَمِعْتُ أَبَا الْقَاسِمِ ﷺ يَقُولُ مَنْ قَذَفَ مَمْلُوكَهُ وَهْوَ بَرِيءٌ مِمَّا قَالَ جُلِدَ يَوْمَ الْقِيَامَةِ إِلاَّ أَنْ يَكُونَ كَمَا قَالَ