Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 186
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 186, with English translation by Maimonides (Rambam).
להרוג אנשי עיר הנידחת ולשרוף את העיר, שנאמר: "ושרפת באש את . . . כל שללה" (דברים יג,יז).
To slay the inhabitants of an apostate city and burn the city, as [Deuteronomy 13:17] states: "And you shall burn the city and all its spoil with fire."