Numbers 3:13-15

American Standard Version

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

¹⁴ And Jehovah spake unto Moses in the wilderness of Sinai, saying, ¹⁵ Number the children of Levi by their fathers’ houses, by their families: every male from a month old and upward shalt thou number them.