Abu Dawud Book 34, Hadith Number 4522.

Chapter : Retaliation for striking and giving retaliation by a ruler on himself.

Narated By Umar ibn al-Khattab : I did not send my collectors (of zakat) so that they strike your bodies and that they take your property. If that is done with someone and he appeals to me, I shall take retaliation on him. Amr ibn al-‘As said: If any man (i.e. governor) inflicts disciplinary punishment on his subjects, would you take retaliation on him too? He said: Yes, by Him in Whose hand my soul is, I shall take retaliation on him. I saw that the Apostle of Allah (PBUH) has given retaliation on himself.