Exodus 13:15 in the King James Version
And it came to pass, when Pharaoh would hardly let us go, that the LORD slew 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 that openeth the matrix, being males; but all the firstborn of my children I redeem.
Context
14 It shall be, when your son asks you in time to come, saying, ‘What is this?’ that you shall tell him, ‘By strength of hand the LORD brought us out from Egypt, from the house of bondage.
15 When Pharaoh stubbornly refused to let us go, the LORD killed all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of livestock. Therefore I sacrifice to the LORD all that opens the womb, being males; but all the firstborn of my sons I redeem.’
16 It shall be for a sign on your hand, and for symbols between your eyes; for by strength of hand the LORD brought us out of Egypt.”