Reference for Chullin 145:43

הרבה מצלת על שאינו גופה יותר מגופה שהרי שנינו

When you say that the slaughtering of a trefah [animal] renders it clean, or [that the slaughtering of an animal renders clean] the loose limb, you are concerned with [the animal] itself, but can it render clean the [limb of the] foetus which is not part of the animal itself?

Jastrow