Exodus 13:11-16
11 "And it shall be, when the LORD brings you into the land of the Canaanites, as He swore to you and your fathers, and gives it to you,
12 "that you shall set apart to the LORD all that open the womb, that is, every firstborn that comes from an animal which you have; the males shall be the LORD'S.
13 "But every firstborn of a donkey you shall redeem with a lamb; and if you will not redeem it, then you shall break its neck. And all the firstborn of man among your sons you shall redeem.
14 "So it shall be, when your son asks you in time to come, saying, 'What is this?' that you shall say to him, 'By strength of hand the LORD brought us out of Egypt, out of the house of bondage.
15 'And it came to pass, when Pharaoh was stubborn about letting us go, that the LORD killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore I sacrifice to the LORD all males that open the womb, but all the firstborn of my sons I redeem.'
16 "It shall be as a sign on your hand and as frontlets between your eyes, for by strength of hand the LORD brought us out of Egypt." (NKJ)