Numbers 3:12-13

Behold, I have taken the Levites from among the Israelites instead of all the firstborn that open the womb among the Israelites. The Levites shall be Mine, because all the firstborn are Mine, for on the day that I struck all the firstborn in the land of Egypt I set apart to Me all the firstborn in Israel, both man and beast. They shall be Mine. I am the LORD.