Musnad Ahmad 30:3357

Ishaq ibn Yusuf narrated to us, Fudayl ibn Ghazwan informed us, from Ibn Abi Nu'm, from Abu Hurayrah, who said: I heard the Prophet of Repentance, may Allah bless him and grant him peace, say: "Any man who accuses his slave of something, while he is innocent of what he said, the prescribed punishment (hadd) will be established upon him on the Day of Resurrection, unless he was as he said."