n°4 / Numbers 3 :

40And 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.
41And 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.