Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrated: We were sitting with the Prophet (peace and blessings be upon him) when a man got up and left, and after he left another man backbit him. So the Prophet (peace and blessings be upon him) said: "Pick your teeth (khilal)." He said: O Messenger of Allah, what should I pick from my teeth, when I have not eaten any meat? He (peace and blessings be upon him) said: "You have eaten the flesh of your brother."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10056