TT #117 · Parshas Shemini · Aliyah 6 · Passuk 9
English: These you may eat among all that live in water: anything in the water, whether in the seas or in the streams, that has fins and scales—these you may eat.
Sifra (Toras Kohanim), Shemini, Section 3.
Sefaria — Sifra, Shemini, Section 3
סְנַפִּיר וְקַשְׂקֶשֶׂת. תַּנְיָא, מִנַּיִן אֲפִלּוּ אִם אֵין לוֹ אֶלָּא סְנַפִּיר אֶחָד וְקַשְׂקֶשֶׂת אַחַת? תַּלְמוּד לוֹמַר סְנַפִּיר וְקַשְׂקֶשֶׂת.
Minimum requirement — The paired terms teach that even one fin and one scale are sufficient for the identifying signs.
A single fin and a single scale can satisfy the signs according to the cited baraita.