Numbers 3:13-15

Darby Translation

¹³ for every firstborn is mine. On the day that I smote all the firstborn in the land of Egypt, I hallowed unto me all the firstborn in Israel, both of man and beast; mine shall they be: I am Jehovah.

¹⁴ And Jehovah spoke to Moses in the wilderness of Sinai, saying,

¹⁵ Number the sons of Levi according to their fathers' houses, after their families; every male from a month old and upward shalt thou number them.