Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 150
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 150, with English translation by Maimonides (Rambam).
לבדוק בסימני העוף עד שיבדיל בין טמא לטהור, שנאמר: "כל ציפור טהורה, תאכלו" (דברים יד,יא).
To check the signs of fowl in order to differentiate between one which is kosher and one which is not kosher, as [Deuteronomy 14:11] states: "All birds which...."