Positive commandment · Mitzvat Aseh #152 of 613
To check the signs [with which] fish [are identified as kosher]
לבדוק בסימני דגים
The verse · Vayikra 11:9
אֶת־זֶה תֹּאכְלוּ מִכֹּל אֲשֶׁר בַּמָּיִם כֹּל אֲשֶׁר־לוֹ סְנַפִּיר וְקַשְׂקֶשֶׂת בַּמַּיִם בַּיַּמִּים וּבַנְּחָלִים אֹתָם תֹּאכֵלוּ
“These shall you eat of all that are in the waters: whatever has fins and scales in the waters, in the seas, and in the rivers, them shall you eat.”
The Rambam’s words
To check the signs [with which] fish [are identified as kosher], as [Leviticus 11:9] states: "These are the animals which you may eat from all that is found in water...."
לבדוק בסימני דגים, שנאמר: "את זה, תאכלו, מכול, אשר במים" (ויקרא יא,ט; דברים יד,ט).
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Forbidden Foods הלכות מאכלות אסורות