Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 152
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 152, with English translation by Maimonides (Rambam).
לבדוק בסימני דגים, שנאמר: "את זה, תאכלו, מכול, אשר במים" (ויקרא יא,ט; דברים יד,ט).
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...."