Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 81
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 81, with English translation by Maimonides (Rambam).
שלא לכבות אש המזבח, שנאמר "אש, תמיד תוקד על המזבח--לא תכבה" (ויקרא ו,ו).
Not to extinguish the fire of the altar, as [Leviticus 6:6] states: "Burn a continuous fire on the altar. Do not extinguish it."