Numbers 3:13-15

Amplified Bible

¹³ for all the firstborn are Mine. On the day that I killed all the firstborn in the land of Egypt, I consecrated for Myself all the firstborn in Israel, both man and animal. They shall be mine; I am the Lord.”

¹⁴ Then the Lord spoke to Moses in the Wilderness of Sinai, saying, ¹⁵ “Number the sons of Levi by their fathers’ households, by their families (clans). You shall number every male from a month old and upward.”