Numbers 3:13-15

King James Version

¹³ Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the Lord.

¹⁴ And the Lord spake unto Moses in the wilderness of Sinai, saying,

¹⁵ Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.