Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 226
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 226, with English translation by Maimonides (Rambam).
להיות בית דין הורגין בסיף, שנאמר: "נקום, יינקם" (שמות כא,כ).
For a court to execute by decapitation, as [Exodus 21:20] states: "Revenge shall surely be taken."