Charles Ellicott Commentary Numbers 18:5-6

"And ye shall keep the charge of the sanctuary, and the charge of the altar; that there be wrath no more upon the children of Israel. And I, behold, I have taken your brethren the Levites from among the children of Israel: to you they are a gift, given unto Jehovah, to do the service of the tent of meeting." — Numbers 18:5-6 (ASV)

And ye shall keep the charge of the sanctuary ... — See Numbers 8:19.