TT #172 · Parshas Shemini · Aliyah 5 · Passuk 31
Chabad.org — Vayikra Chapter 11 with Rashi
English: These are the impure among the swarming things; whoever touches them when dead shall be impure until evening.
Chagigah 11a.
E-Daf.com — Chagigah 11a (Tsurras Hadaf)
Textual measure — The contrasting words “them” and “from them” establish that a qualifying portion can represent the whole, and the sages define its measure.
Contact with a sufficient portion of a sheretz conveys impurity; the sages establish the measure as a lentil's size.