Abu Dawud Book 18, Hadith Number 3566.

Narated By Buraydah ibn al-Hasib : The Prophet (PBUH) said: Judges are of three types, one of whom will go to Paradise and two to Hell. The one who will go to Paradise is a man who knows what is right and gives judgment accordingly; but a man who knows what is right and acts tyrannically in his judgment will go to Hell; and a man who gives judgment for people when he is ignorant will go to Hell.