Exodus 11:4-5
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4And Moses said, "Thus says the LORD: About midnight I will go forth in the midst of Egypt; 5and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh who sits upon his throne, even to the first-born of the maidservant who is behind the mill; and all the first-born of the cattle.
Exodus 11:4-5 (RSV)