Numbers 8:15-17

Amplified Bible

¹⁵ Then after that the Levites may go in to serve the Tent of Meeting (tabernacle), but you shall cleanse them [first] and present them as a wave offering; ¹⁶ for they are wholly given to Me from among the Israelites. I have taken them for Myself instead of ^[a]all who are born first, the firstborn of all the Israelites. ¹⁷ For all the firstborn among the Israelites are Mine, among the men and among the animals; on the day that I struck down every firstborn in the land of Egypt, I sanctified and set the Israelites apart for Myself.