And when Pharaoh stubbornly refused to let us go, the LORD killed every firstborn in the land of Egypt, both of man and beast. This is why I sacrifice to the LORD the firstborn male of every womb, but I redeem all the firstborn of my sons.’
Jewish Publication Society of America Version (JPS1917) - Exodus 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 first-born of man, and the first-born of beast; therefore I sacrifice to the LORD all that openeth the womb, being males; but all the first-born of my sons I redeem.