Exodus 13:15

13:15 and it happened, when Pharaoh would hardly let us go, that Yahweh killed all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of animal. Therefore I sacrifice to Yahweh all that opens the womb, being males; but all the firstborn of my sons I redeem.'
-World English Bible

Other Translation

13:15 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.
-King James Version