Exodus 13:14-15

MLV(i) 14 And it will be, when your son asks you in time to come, saying, What is this? that you will say to him, By strength of hand Jehovah brought us out from Egypt, from the house of bondage.
15 And it happened, when Pharaoh would hardly let us go, that Jehovah killed* all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore I sacrifice to Jehovah all that opens the womb, being males, but all the firstborn of my sons I redeem.