TT #191 · Parshas Shemini · Aliyah 7 · Passuk 33
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Into its midst”: an earthen vessel without an interior cavity has no back-side status.
Toras Kohanim.
Wikisource — Sifra, Shemini, Chapter 7
Textual inference — The Chazal derives the stated rule directly from the wording of the verse.
An earthenware object without an interior lacks the corresponding back impurity.