Numbers 8:14-17

14In this way you are to set apart the Levites from Bnei-Yisrael to be Mine.

15After you have purified them and presented them as a wave offering, the Levites will come to do their work at the Tent of Meeting.

16For they are the ones from among Bnei-Yisrael given to Me in place of all the first from the wombs of Bnei-Yisrael. I have taken them for Myself.

17“For every firstborn among Bnei-Yisrael is Mine, whether human or animal. On the day I struck down the firstborn of the land of Egypt, I sanctified them for Myself.