Numbers 3:40 (KJV)

But those that encamp before the tabernacle toward the east, even before the tabernacle of the congregation eastward, shall be Moses, and Aaron and his sons, keeping the charge of the sanctuary for the charge of the children of Israel; and the stranger that cometh nigh shall be put to death.

All that were numbered of the Levites, which Moses and Aaron numbered at the commandment of the Lord, throughout their families, all the males from a month old and upward, were twenty and two thousand.

And the Lord said unto Moses, Number all the firstborn of the males of the children of Israel from a month old and upward, and take the number of their names.

And thou shalt take the Levites for me (I am the Lord) 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.

And Moses numbered, as the Lord commanded him, all the firstborn among the children of Israel.