Numbers 8:19 (BSB)

And I have given the Levites as a gift to Aaron and his sons from among the Israelites, to perform the service for the Israelites at the Tent of Meeting and to make atonement on their behalf, so that no plague will come against the Israelites when they approach the sanctuary.”

From Numbers 8. Also in the ESV.

Commentary on Numbers 8:19

• John Calvin (Reformed), Calvin's Commentaries on Numbers 8:19: 19. And I have given the Levites. He declares on what terms He desires to have them as His own, viz., that they may be directed by Aaron, and obey his commands; for by “a gift” is not to be understood such an act as that whereby a person resigns and cedes his own right to another; but, when He devotes them to the ministry...
• Matthew Henry (Presbyterian), Matthew Henry's Complete Commentary on Numbers 8:5-26: We read before of the separating of the Levites from among the children of Israel when they were numbered, and the numbering of them by themselves (Num 3:6, Num 3:15), that they might be employed in the service of the tabernacle. Now here we have directions given for their solemn ordination (Num 8:6), and the performance of it, Num 8:20.
• John Gill (Reformed Baptist), Exposition of the Old and New Testaments on Numbers 8:19: And Moses and Aaron, and all the congregation of the children of Israel,.... The several parties concerned in the consecration of the Levites: these did to the Levites according unto all that the Lord commanded Moses, so did the children of Israel unto them: which, as Jarchi notes, is said to the praise of them that did it, and of what was done by them...
• Jamieson-Fausset-Brown (Reformed), Commentary Critical and Explanatory on the Whole Bible on Numbers 8:19: to make an atonement for the children of Israel, &c.--to aid the priests in that expiatory work; or, as the words may be rendered, "to make redemption for" the Levites being exchanged or substituted for the first-born for this important end, that there might be a sanctified body of men appointed to guard the sanctuary, and the people not allowed to approach or presumptuously meddle...