Positive commandment · Mitzvat Aseh #97 of 613
Impurity of the eight swarming creatures
להיות שמונה שרצים מטמאין
The verse · Vayikra 11:29
וְזֶה לָכֶם הַטָּמֵא בַּשֶּׁרֶץ הַשֹּׁרֵץ עַל־הָאָרֶץ הַחֹלֶד וְהָעַכְבָּר וְהַצָּב לְמִינֵהוּ
“These also shall be unclean to you among the creeping things that creep upon the earth; the rat, and the mouse, and the tortoise after its kind.”
The Rambam’s words
For the [dead bodies of] the eight species of crawling animals [mentioned in the Torah] to impart ritual impurity, as [Leviticus 11:19] states: "These shall be impure for you...."
להיות שמונה שרצים מטמאין, שנאמר "וזה לכם הטמא" (ויקרא יא, כט).
Why · Sefer HaChinuch #159
Impurity of the eight swarming creatures
“From the roots of the commandment, [we need to] preface [that] anyone who has intelligence will not have a doubt that there was never a man that grasped to know all wisdom to its end, that nothing of it be hidden from him.”
Who & when
“And [it] is practiced in every place and at all times by males and females, regarding that anyone who is rendered impure by swarming creatures is called impure and does not emerge from his impurity until he immerses in water, like his law.”
Where in the Mishneh Torah
Hilchot Other Sources of Defilement הלכות שאר אבות הטומאות