Then the LORD said to Moses: "Number all the firstborn males of the children of Israel from a month old and above, and take the number of their names.
Numbers 3:40
Surrounding context
3:39 All who were numbered of the Levites, whom Moses and Aaron numbered at the commandment of the LORD, by their families, all the males from a month old and above, were twenty-two thousand.
3:40 Then the LORD said to Moses: "Number all the firstborn males of the children of Israel from a month old and above, and take the number of their names.
3:41 And you 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."