TT #221 · Parshas Shemini · Aliyah 7 · Passuk 38
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Upon seed”: just as seed does not ordinarily transmit the most severe form of impurity, so other items that do not transmit that severe impurity require preparation through liquid.
Chullin 121a.
E-Daf.com — Chullin 121a (Tsurras Hadaf)
Analogy — The Chazal derives the stated rule from the wording and structure of the verse.
The preparation rule extends to things not destined for severe impurity.