Numbers 3:41-45

41And thou shalt take the Levites for me (I am Yhwh) instead of all the firstborn among the children of Israel; and the cattle of the Levites instead of all the firstlings among the cattle of the children of Israel. 42And Moses numbered, as Yhwh commanded him, all the firstborn among the children of Israel. 43And all the firstborn males by the number of names, from a month old and upward, of those that were numbered of them, were twenty and two thousand two hundred and threescore and thirteen.

44And Yhwh spake unto Moses, saying, 45Take the Levites instead of all the firstborn among the children of Israel, and the cattle of the Levites instead of their cattle; and the Levites shall be mine: I am Yhwh.