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. 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 firstborn animal was set apart for God. Every firstborn son among the children was to be redeemed by a payment, a ransom owed to God. That ransom stood among the regular claims and returns owed to God, keeping the obligation continually before the people. The firstborn and first-fruits offerings taught Israel to put God first and to acknowledge that their fields, flocks, herds, possessions, and children came from Him and belonged to Him. The offerings presented at the sanctuary under this requirement were devoted to the priests.