Exodus 11:4-5
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4Moses said, "This is what Yahweh says: `About midnight I will go out into the midst of Egypt, 5and all the firstborn in the land of Egypt shall die, from the firstborn of Pharaoh who sits on his throne, even to the firstborn of the maid-servant who is behind the mill; and all the firstborn of cattle.
Exodus 11:4-5 (WEB)