Sahih Muslim Book 16, Hadith Number 4143.

Chapter : When anyone attacks the life of a person, or his limb and the victim wards off the attack and in self-defence either the life of the assailant is lost or his limb broken, there would be no penalty on the victim.

Imran b. Husain reported: Ya’la b. Munya or Ibn Umayya fought with a person, and the one bit the hand of the other. And he tried to draw his hand from his mouth and thus his fore-teeth ware pulled out. They referred their dispute to Allah’s Apostle (may peace be upon him), whereupon he said: Does any one of you bite as the camel bites? So there is no blood-wit for it.