Numbers 3:13-15

Young's Literal Translation

¹³ for Mine [is] every first-born, in the day of My smiting every first-born in the land of Egypt I have sanctified to Myself every first-born in Israel, from man unto beast; Mine they are; I [am] Jehovah.'

¹⁴ And Jehovah speaketh unto Moses in the wilderness of Sinai, saying,

¹⁵ `Number the sons of Levi by the house of their fathers, by their families; every male from a son of a month and upward thou dost number them.'