Positive commandment · Mitzvat Aseh #29 of 613
To keep a fire burning on the altar for burnt offerings continually
לַעֲרֹךְ אֵשׁ בְּמִזְבַּח הָעוֹלָה תָּמִיד
The verse · Vayikra 6:6
אֵשׁ תָּמִיד תּוּקַד עַל־הַמִּזְבֵּחַ לֹא תִכְבֶּה
“The fire shall ever be burning upon the altar; it shall never go out.”
The Rambam’s words
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."
לַעֲרֹךְ אֵשׁ בְּמִזְבַּח הָעוֹלָה תָּמִיד, שֶׁנֶּאֱמָר: "אֵשׁ תמיד תוקד על המזבח" (ויקרא ו, ו).
Why · Sefer HaChinuch #132
Lighting fire on the altar every day
“From the roots of the commandment, [we need to] preface [that] the thing is known amongst us and among every sage that the great miracles that G-d does for people in His great goodness, He always does hiddenly.”
Who & when
“And it is practiced at the time of the Temple by the males of the priesthood.”
Where in the Mishneh Torah
Hilchot Daily Offerings and Additional Offerings הלכות תמידים ומוספין