Positive commandment · Mitzvat Aseh #107 of 613
For a corpse to impart impurity
להיות המת מטמא
The verse · Bamidbar 19:2
זֹאת חֻקַּת הַתּוֹרָה אֲשֶׁר־צִוָּה ה׳ לֵאמֹר דַּבֵּר אֶל־בְּנֵי יִשְׂרָאֵל וְיִקְחוּ אֵלֶיךָ פָרָה אֲדֻמָּה תְּמִימָה אֲשֶׁר אֵין־בָּהּ מוּם אֲשֶׁר לֹא־עָלָה עָלֶיהָ עֹל
“This is the ordinance of the Tora which the Lord has commanded, saying, Speak to the children of Yisra᾽el, that they bring thee a red heifer without defect, in which there is no blemish, and upon which never came a yoke:”
The Rambam’s words
For a corpse to impart impurity.
להיות המת מטמא.
Why · Sefer HaChinuch #398
Impurity of a dead body
“It is from the roots of the commandment [that] since the priests were chosen for the service of G-d, blessed be He — as the verse stated, “they shall be holy to their G-d” (Leviticus 21:6) — therefore, He distanced them from the dead.”
Where in the Mishneh Torah
Hilchot Defilement by a Corpse הלכות טומאת מת