Sahih Muslim Book 020, Hadith Number 4504.
Chapter : The merits of a just ruler and the demerits of a tyrant ruler.
It has been narrated on the authority of Hasan that A'idh b. 'Amr who was one of the Companions of the Messenger of Allah (may peace be upon him) called on 'Ubaidullah b. Ziyad and said (to him): O my son, I have heard the Messenger of Allah (may peace be upon him) say: The worst of guardians is the cruel ruler. Beware of being one of them. Ubaidullah said (to him out of arrogance): Sit you down. You are from the chaff of the Companions of Muhammad (may peace be upon him). A'idh said: Was there worthless chaff among them? Such worthless chaff appeared after them and among other people.