Exodus 27:20-30

20 "You shall command the children of Yisra'el, that they bring to you pure olive oil beaten for the light, to cause a lamp to burn continually.
21 In the tent of meeting, outside the veil which is before the testimony, Aharon and his sons shall keep it in order from evening to morning before the LORD: it shall be a statute forever throughout their generations on the behalf of the children of Yisra'el.