Positive commandment · Mitzvat Aseh #181 of 613
To decapitate the calf [brought as atonement for an unsolved murder] as required
לערוף את העגלה כמצותה
The verse · Devarim 21:4
וְהוֹרִדוּ זִקְנֵי הָעִיר הַהִוא אֶת־הָעֶגְלָה אֶל־נַחַל אֵיתָן אֲשֶׁר לֹא־יֵעָבֵד בּוֹ וְלֹא יִזָּרֵעַ וְעָרְפוּ־שָׁם אֶת־הָעֶגְלָה בַּנָּחַל
“and the elders of that city shall bring down the heifer to a rough ravine, which is neither ploughed nor sown, and shall break the heifer’s neck there in the ravine:”
The Rambam’s words
To decapitate the calf [brought as atonement for an unsolved murder] as required, as [Deuteronomy 21:4] states: "You shall decapitate the calf there in the river."
לערוף את העגלה כמצותה, שנאמר: "וערפו שם את העגלה, בנחל" (דברים כא,ד).
Why · Sefer HaChinuch #530
To behead the calf in the riverbed
“It is from the roots of the commandment [that it is] in order that the heart of the people be aroused in their seeing this great procedure — the gathering of the elders of the city and its great men: And they take a cow, which is a large animal, and they go outside of the city in assembly, and with the masses — since everyone wants to see these things.”
Who & when
“And this commandment is practiced in the Land of Israel at the time when it is in its inhabitation, and also in Transjordan.”
Where in the Mishneh Torah
Hilchot Murderer and the Preservation of Life הלכות רוצח ושמירת נפש