Exodus 13:13

And every firstling of an ass thou shalt redeem with a lamb; and if thou wilt not redeem it, then thou shalt break his neck: and all the firstborn of man among thy children shalt thou redeem.

Every firstling ... The injunction respecting the consecration of the first-born is here repeated with some additional circumstances. The firstlings of clean beasts, such as lambs, kids, and calves, if males, were to be devoted to God and employed in sacrifice. Unclean beasts, such as the donkey's colt, being unfit for sacrifice, were to be redeemed (Numbers 18:15). If not redeemed, the neck or backbone was to be broken. In Babylonia, dogs were in this manner devoted to the Tyrian Hercules (cf. Isaiah 66:3; Bunsen's 'Egypt,' 4:, 213).

Thus, there is a double record of the exodus-namely, the feast of unleavened bread, in remembrance of the day on which they departed, and the consecration of the first-born to the Lord, in memory of the destruction of the first-born of the Egyptians on the previous night (Numbers 8:17). The dedication of all the first-born of the herds to God was not an institution for which the singular reason just mentioned was arbitrarily assigned at a remote period from its original, but it was assigned at the time it took place, to be perpetually recorded as the true cause (Graves 'On the Pentateuch,' vol. 1:, p. 222). At a subsequent stage of the Theocracy, this latter was modified, though not repealed (Numbers 3:15), and the remembrance of the Lord's claim was perpetuated by the enactment respecting the redemption of the first born (see the note at Numbers 18:16).