Positive commandment · Mitzvat Aseh #98 of 613
For [certain] foods to impart impurity
להיות האוכלין מיטמאין
The verse · Vayikra 11:34
מִכׇּל־הָאֹכֶל אֲשֶׁר יֵאָכֵל אֲשֶׁר יָבוֹא עָלָיו מַיִם יִטְמָא וְכׇל־מַשְׁקֶה אֲשֶׁר יִשָּׁתֶה בְּכׇל־כְּלִי יִטְמָא
“Of all food which may be eaten, that on which water comes shall be unclean: and all drink that may be drunk in every vessel shall be unclean.”
The Rambam’s words
For [certain] foods to impart impurity, as [Leviticus 11:34] states: "From all the food which you will eat...."
להיות האוכלין מיטמאין, שנאמר "מכל האוכל אשר יאכל" (ויקרא יא, לד).
Who & when
“And [it] is practiced in every place and at all times by males and females — that we determine the impurity of foods and drinks in this manner.”
Where in the Mishneh Torah
Hilchot Defilement of Foods הלכות טומאת אוכלים