Numbers 18:15
Every thing that openeth the matrix in all flesh, which they bring unto the LORD, whether it be of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.
Every firstborn of both people and animals belonged to the Lord as His own by special claim. The firstborn son was not to be given up in service or sacrifice but was to be bought back by the payment of a ransom price. That redemption stood as a continual acknowledgment that Israel's firstborn had been spared on the night of the Passover judgment. Though they too had been exposed to death with the rest, their lives had been preserved by God's mercy through the atoning sacrifice.