Numbers 8:17-19

17For every firstborn among the Israelites is mine, both man and animal. I consecrated them to myself on the day I struck down every firstborn in the land of Egypt.

18But I have taken the Levites in place of every firstborn among the Israelites.

19From the Israelites, I have given the Levites exclusively to Aaron and his sons to perform the work 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.”