Punishment of Disbelievers at War with Allah and His Apostle

Bukhari :: Book 8 :: Volume 82 :: Hadith 841

Narrated Abu Huraira:

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."