Sahih Muslim Book 16, Hadith Number 4146.

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.

Safwan b. Ya’la reported that a person bit the arm of the servant of Ya’la b. Munya. He pulled it and his fore-tooth fell. The matter was referred to Allah’s Apostle (may peace be upon him) and he turned it down and said: Did you intend to bite his hand, as the camel bites?