Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 181
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 181, with English translation by Maimonides (Rambam).
לערוף את העגלה כמצותה, שנאמר: "וערפו שם את העגלה, בנחל" (דברים כא,ד).
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."