If somebody slanders his slave and the slave is innocent of what he says, he (the master) will be flogged on the Day of Judgment unless the slave really is as he described him. [Al-Bukhari]

"«من قَذَفَ مَمْلُوكَهُ، وهو بَرِيءٌ مما قال جُلِد يوم القيامة إلا أن يكون كما قال». [صحيح.] - [متفق عليه.]"