TT #129 · Parshas Shemini · Aliyah 6 · Passuk 12
Chabad.org — Vayikra Chapter 11 with Rashi
English: Everything in the water that has fins and scales, in the seas or in the streams, you may eat.
Sifra (Toras Kohanim), Shemini, Section 3.
Sefaria — Sifra, Shemini, Section 3
סְנַפִּיר וְקַשְׂקֶשֶׂת בַּמַּיִם. מַה תַּלְמוּד לוֹמַר? שֶׁיָּכוֹל אֵין לִי אֶלָּא הַמַּעֲלֶה סִימָנֵי טָהֳרָה שֶׁלּוֹ לַיַּבָּשָׁה, הִשִּׁירָן בַּמַּיִם מִנַּיִן? תַּלְמוּד לוֹמַר בַּמַּיִם.
Textual clarification — “In the water” teaches that the identifying signs must remain present in the water; removing them on land does not satisfy the condition.
If the fish possessed its kosher signs while in the water, later shedding them does not retroactively make it prohibited.