Exodus 11:4-5

⁴And Moses said, Thus saith Jehovah, About midnight will I go out into the midst of Egypt: ⁵and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne, even unto the first-born of the maid-servant that is behind the mill; and all the first-born of cattle.