(19, 20) باب الرَّجُلِ يَدْفَعُ عَنْ نَفْسِهِ،
It was narrated from Ya'la bin Munyah that:
he fought a man and one of them bit the other, who pulled his forearm away from his mouth, and a front tooth fell out. The matter was referred to the Prophet and he said: "Would one of you bite his brother as a young camel bites?" And judged it to be invalid.
Reference: Sunan an-Nasa'i 4763
In-book reference: Book 45, Hadith 58
English translation: Vol. 5, Book 45, Hadith 4767