Reference for Chullin 256:25

אי הכי אפילו בשר נמי

Then in that case even flesh [severed from the living animal] should also [render unclean], should it not? - You cannot say so, for it has been taught: I might think that flesh severed from the living animal should also be unclean, Scripture therefore states: And if there die of the beasts: as death cannot be replaced so everything that [is severed and] cannot be replaced [renders unclean]; so R'Jose [the Galilean].

Jastrow