TT #188 · Parshas Shemini · Aliyah 7 · Passuk 33
Chabad.org — Vayikra Chapter 11 with Rashi
English: “That falls from them”: this excludes a chicken that swallowed a creeping creature and then fell into the airspace of an oven; while it remains alive the oven is pure, but if it dies the oven is impure.
Toras Kohanim.
Wikisource — Sifra, Shemini, Chapter 7
Case distinction — The Chazal derives the stated rule directly from the wording of the verse.
The live swallowed-sheretz case differs from the dead case.