Numbers 3:18 (KJV)

Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.

And the LORD spake unto Moses in the wilderness of Sinai, saying,

Number the children of Levi after the house of their fathers, by their families: every male from a month old and upward shalt thou number them.

And Moses numbered them according to the word of the LORD, as he was commanded.

And these were the sons of Levi by their names; Gershon, and Kohath, and Merari.

And the sons of Kohath by their families; Amram, and Izehar, Hebron, and Uzziel.

And the sons of Merari by their families; Mahli, and Mushi. These are the families of the Levites according to the house of their fathers.

Of Gershon was the family of the Libnites, and the family of the Shimites: these are the families of the Gershonites.

Those that were numbered of them, according to the number of all the males, from a month old and upward, even those that were numbered of them were seven thousand and five hundred.

The families of the Gershonites shall pitch behind the tabernacle westward.