כִּי־יִמָּצֵ֣א חָלָ֗ל בָּאֲדָמָה֙ אֲשֶׁר֩ יְהוָ֨ה אֱלֹהֶ֜יךָ נֹתֵ֤ן לְךָ֙ לְרִשְׁתָּ֔הּ נֹפֵ֖ל בַּשָּׂדֶ֑ה לֹ֥א נוֹדַ֖ע מִ֥י הִכָּֽהוּ׃
אַז אַ געהרגעטער װעט געפֿונען װערן ליגנדיק אין פֿעלד, אין דעם לאַנד װאָס ה׳ דײַן גאָט גיט דיר עס צו אַרבן, און עס איז ניט באַװוּסט װער עס האָט אים דערשלאָגן,
Sefer HaMitzvot
That is that He commanded us to behead a calf according to its commandment when a corpse is found in the field and it is not known who killed him. And that is His, may He be exalted, saying, "If you find a corpse, etc." (Deuteronomy 21:1). And this is the law of the beheaded calf. And the regulations of this commandment have already been explained in the last chapter of Sotah. (See Parashat Shoftim; Mishneh Torah, Murderer and the Preservation of Life 8.)
Mishneh Torah, Positive Mitzvot
To decapitate the heifer in the manner prescribed, as it is said, “…. and they shall break off the heifer’s neck in the valley” (Deut. 21:4).