Numbers 3:13-17

For all the firstborn are Mine; on the day that I fatally struck all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, from the human firstborn to animals. They shall be Mine; I am the Lord.”

Then the Lord spoke to Moses in the wilderness of Sinai, saying,

“Count the sons of Levi by their fathers’ households, by their families; every male from a month old and upward you shall count.”

So Moses counted them according to the word of the Lord, just as he had been commanded.

These, then, are the sons of Levi by their names: Gershon, Kohath, and Merari.