Torath Kohanim / Yoma material already linked by TT under the continuous-fire verses (reinforced here).
אש תמיד תוקד על המזבח לא תכבה — מצוה שתהא האש דולקת תמיד, ואפילו בשבת ואפילו בטומאה.
“A continuous fire shall burn upon the altar; it shall not go out” — it is a mitzvah that the fire burn continuously, even on Shabbat and even in a state of impurity.
Custom — reinforcement of the perpetual-fire commandment.
The altar fire must burn without interruption at all times.