Exodus 27:20-21

20“You shall command the Israelites to provide you with clear oil of beaten olives for the light, to make a lamp burn continually [every night].

21In the Tent of Meeting [of God with His people], outside the veil which is in front of the [ark of the] Testimony [and sets it apart], Aaron [the high priest] and his sons shall keep the lamp burning from evening to morning before the Lord. It shall be a perpetual statute [to be observed] throughout their generations on behalf of the Israelites.