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.
Berean Study Bible (BSB) - Numbers 3:13 >for all the firstborn are Mine. On the day I struck down every firstborn in the land of Egypt, I consecrated to Myself all the firstborn in Israel, both man and beast. They are Mine; I am the LORD.”