Exodus 13:2

2 aConsecrate to me all the firstborn. Whatever is the first to open the womb among the people of Israel, both of man and of beast, is mine.”

Exodus 13:15

15For when Pharaoh stubbornly refused to let us go, the bLord killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of animals. Therefore I sacrifice to the Lord all the males that first open the womb, but call the firstborn of my sons I redeem.’
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