Exodus 4:23; Exodus 11:5

23 and I say to you, Let my son go that he may serve me. If you refuse to let him go, behold, I ewill kill your firstborn son.

and every firstborn in the land of Egypt shall die, from the firstborn of Pharaoh who sits on his throne, even to the firstborn of the slave girl who is xbehind the handmill, and all the firstborn of the cattle.