Hadith: If somebody slanders his slave and the slave is innocent of what he says, he will be flogged on the Day of Judgment unless the slave really is as he described him

عن أبي هريرة رضي الله عنه قال: سمعت أبا القاسم صلى الله عليه وسلم يقول: «من قَذَفَ مَمْلُوكَهُ، وهو بَرِيءٌ مما قال جُلِد يوم القيامة إلا أن يكون كما قال».
[صحيح] - [متفق عليه]

Abu Hurayrah (may Allah be pleased with him) reported: I heard the Prophet (may Allah's peace and blessings be upon him) say: "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."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

If a master slanders his slave, no punishment is to be inflicted upon him in the worldly life, for corporeal punishments expiate the sins. Given that the punishment will be inflicted upon him in the Hereafter, he is not to be punished in the worldly life. The scholars unanimously agree that the prescribed corporal punishment is not to be inflicted on the slave in such a case. The master is not to be subject to the prescribed corporal punishment of slander either, for he slanders him only when he is certain or has a strong assumption. The accusation brings down the price of the slave, which involves loss to the master. Finally, this Hadīth specifies the general indication of Allah's saying: {Those who slander the chaste women and do not bring four witness, flog them eighty lashes.} [Sūrat An-Nūr: 4]