Exodus 27:20-21

20And thou shalt command the children of Israel, that they bring thee olive oil, pure, beaten, for the light, to light the lamp continually. 21In the tent of meeting outside the veil, which is before the testimony, Aaron and his sons shall dress them from evening to morning before Jehovah: [it is] an everlasting statute, for their generations, on the part of the children of Israel.