Exodus 4:23

4:23 and I said to you, ‘Let my son go that he may serve me,’ but since you have refused to let him go, I will surely kill your son, your firstborn!”’”

Exodus 11:5

11:5 and all the firstborn in the land of Egypt will die, from the firstborn son of Pharaoh who sits on his throne, to the firstborn son of the slave girl who is at her hand mill, and all the firstborn of the cattle.