Reference for Chullin 256:18

אבר מן החי דמטמא מאי קרא

What difference is there between a limb torn from a living animal and a limb torn from a dead animal? - The difference is where some flesh is severed from the limb; for flesh severed from the limb torn from a living animal is not rendered unclean, but [flesh severed] from the limb torn from a dead animal is rendered unclean.

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