Numbers 3:40

38Those who encamp before the tabernacle eastward, in front of the Tent of Meeting toward the sunrise, shall be Moses, with Aaron and his sons, keeping the requirements of the sanctuary for the duty of the children of Israel. The outsider who comes near shall be put to death.

39All who were counted of the Levites, whom Moses and Aaron counted at the commandment of the LORD, by their families, all the males from a month old and upward, were twenty-two thousand.

40The LORD said to Moses, “Count all the firstborn males of the children of Israel from a month old and upward, and take the number of their names.

41You shall take the Levites for me—I am the LORD—instead of all the firstborn among the children of Israel; and the livestock of the Levites instead of all the firstborn among the livestock of the children of Israel.”

42Moses counted, as the LORD commanded him, all the firstborn among the children of Israel.