Sahih Muslim Book 16, Hadith Number 4147.

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 that a person bit the hand of a person. He withdrew his hand and his fore-tooth or fore-teeth fell down. He (the man who lost his teeth) referred the matter to Allah’s Messenger (may peace be upon him) and he said, What do you want me to do? Do you ask me that I should order him to put his hand in your month, and you should bite it as the camel bites? (If you want retaliation, then the only way out is) that you put your hand in his mouth (allow him) to bite that and then draw it away.