Numbers 3:41

3:41 And take the Levites for me – I am the Lord – instead of all the firstborn males among the Israelites, and the livestock of the Levites instead of all the firstborn of the livestock of the Israelites.”

Numbers 8:17

8:17 For all the firstborn males among the Israelites are mine, both humans and animals; when I destroyed all the firstborn in the land of Egypt I set them apart for myself.