Exodus 13:2

Sanctify unto Me all the first-born, whatsoever openeth the womb among the children of Israel, both of man and of beast; it is Mine. This is not said of the general sanctification of the people as God's own nation, but of the consecrating, of the setting apart, of the first-born, both of men and of animals, for the specific service of Jehovah. They were the Lord's not only by virtue of creation, but because He spared them in the great slaughter in Egypt. The first-born sons were to serve the Lord as priests, until this right was vested in the descendants of Levi, when they were obliged to redeem themselves by payment of a sacrifice of exemption; the first-born beasts were to be sacrificed to the Lord.