سنن النسائي

It was narrated from Safwan bin Ya'la that: his father wen on the campaign of Tabuk with the Messenger of Allah, and he hired a man who fought with another man. The man bit his forearm, and when it hurt him, he pulled it away, and the man's front tooth fell out. The matter was referred to the Messenger of Allah (ﷺ) who said: "Would one of you deliberately bit his brother as a stallion bites?" And he judged it to be invalid.