Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 29
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 29, with English translation by Maimonides (Rambam).
לַעֲרֹךְ אֵשׁ בְּמִזְבַּח הָעוֹלָה תָּמִיד, שֶׁנֶּאֱמָר: "אֵשׁ תמיד תוקד על המזבח" (ויקרא ו, ו).
To keep a fire burning on the altar for burnt offerings continually, as [Leviticus 6:6] states: "And you shall keep a fire burning continuously on the altar."