Numbers 3:13 in the King James Version
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.
Context
12 “Behold, I have taken the Levites from among the children of Israel instead of all the firstborn who open the womb among the children of Israel; and the Levites shall be mine,
13 for all the firstborn are mine. On the day that I struck down all the firstborn in the land of Egypt I made holy to me all the firstborn in Israel, both man and animal. They shall be mine. I am the LORD.”
14 The LORD spoke to Moses in the wilderness of Sinai, saying,