Exodus 11:4-5
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4And Moses said, Thus saith the LORD, About midnight will I go out into the midst of Egypt: 5And all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh that sitteth upon his throne, even unto the firstborn of the maidservant that is behind the mill; and all the firstborn of beasts.
Exodus 11:4-5 (KJV)