TT #196 · Parshas Shemini · Aliyah 7 · Passuk 33
Chabad.org — Vayikra Chapter 11 with Rashi
English: “It shall become impure”: Rabbi Akiva derives that the impure food can transmit impurity onward, teaching the rule of a second degree of impurity transmitting to a third.
Sotah 27b.
E-Daf.com — Sotah 27b (Tsurras Hadaf)
Textual derivation — The Chazal derives the stated rule directly from the wording of the verse.
Rabbi Akiva derives the applicable second-to-third degree rule.