Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 228
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 228, with English translation by Maimonides (Rambam).
להיות בית דין שורפין באש, שנאמר: "באש ישרפו אותו, ואתהן" (ויקרא כ,יד).
For a court to execute by burning [the condemned] with fire, as [Leviticus 20:14] states: "They shall burn him and them with fire."