Abu Dawud Book 34, Hadith Number 4521.

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

Narated By AbuSa’id al-Khudri : When the Apostle of Allah (PBUH) was distributing something, a man came towards him and bent down on him. The Apostle of Allah (PBUH) struck him with a bough and his face was wounded. The Apostle of Allah (PBUH) said to him: Come and take retaliation. He said: no, I have forgiven, Apostle of Allah!