Thematic Bible: Firstborn set apart as a

that thou shalt set apart unto the LORD all that opens the womb and in the same manner every firstborn that opens the wombs of thy animals; the males shall be the LORD's. And every firstborn of an ass thou shalt ransom with a lamb; and if thou wilt not ransom it, then thou shalt cut off its head; and in the same manner thou shalt ransom all the human firstborn among thy sons. And it shall be when thy son asks thee in time to come, saying, What is this? Thou shalt say unto him, With a strong hand the LORD brought us out of Egypt, from the house of slavery; read more.
and it came to pass when Pharaoh was hardening himself to not let us go that the LORD slew all the firstborn in the land of Egypt from the human firstborn to the firstborn of the beast; therefore, I sacrifice to the LORD every male that opens the womb, and I ransom every firstborn of my sons. And it shall be for a token upon thy hand, and for a memorial before thine eyes: That the LORD brought us out of Egypt with a strong hand.