And the 25th mitzvah is that the priests are commanded to light the lamps [of the menorah] continually before G‑d [i.e. in the Communion Tent].

The source of this commandment is G‑d's statement1 (exalted be He), "Aaron and his sons shall light it in the Communion Tent, outside the cloth partition that conceals the [Ark of] Testimony." This is known as hatavas haneiros.

The details of this mitzvah have been explained in the 8th chapter of Menachos,2 in the 1st chapter of Yoma,3 and in several places in tractate Tamid.4