Numbers 3:13

because all the firstborn are mine. When I destroyed all the firstborn in the land of Egypt, I set apart for myself all the firstborn in Israel, both man and beast. They belong to me. I am the Lord."

Exodus 13:2

"Set apart to me every firstborn male -- the first offspring of every womb among the Israelites, whether human or animal; it is mine."

Exodus 13:12

then you must give over to the Lord the first offspring of every womb. Every firstling of a beast that you have -- the males will be the Lord's.

Exodus 13:15

When Pharaoh stubbornly refused to release us, the Lord killed all the firstborn in the land of Egypt, from the firstborn of people to the firstborn of animals. That is why I am sacrificing to the Lord the first male offspring of every womb, but all my firstborn sons I redeem.'

Leviticus 27:26

"'Surely no man may consecrate a firstborn that already belongs to the Lord as a firstborn among the animals; whether it is an ox or a sheep, it belongs to the Lord.

Exodus 12:29-30

It happened at midnight -- the Lord attacked all the firstborn in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the captive who was in the prison, and all the firstborn of the cattle.

Exodus 22:29

"Do not hold back offerings from your granaries or your vats. You must give me the firstborn of your sons.

Exodus 34:19

"Every firstborn of the womb belongs to me, even every firstborn of your cattle that is a male, whether ox or sheep.

Numbers 8:16-17

For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites.

Numbers 18:15

The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, and the firstborn males of unclean animals you must redeem.

Ezekiel 44:30

The first of all the first fruits and all contributions of any kind will be for the priests; you will also give to the priest the first portion of your dough, so that a blessing may rest on your house.

Luke 2:23

(just as it is written in the law of the Lord, "Every firstborn male will be set apart to the Lord"),

Hebrews 12:23

and congregation of the firstborn, who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous, who have been made perfect,

Treasury of Scripture Knowledge

Summary

Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.

Bible References

Because

Numbers 8:16
For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites.
Numbers 18:15
The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, and the firstborn males of unclean animals you must redeem.
Exodus 13:2
"Set apart to me every firstborn male -- the first offspring of every womb among the Israelites, whether human or animal; it is mine."
Exodus 22:29
"Do not hold back offerings from your granaries or your vats. You must give me the firstborn of your sons.
Exodus 34:19
"Every firstborn of the womb belongs to me, even every firstborn of your cattle that is a male, whether ox or sheep.
Leviticus 27:26
"'Surely no man may consecrate a firstborn that already belongs to the Lord as a firstborn among the animals; whether it is an ox or a sheep, it belongs to the Lord.
Ezekiel 44:30
The first of all the first fruits and all contributions of any kind will be for the priests; you will also give to the priest the first portion of your dough, so that a blessing may rest on your house.
Luke 2:23
(just as it is written in the law of the Lord, "Every firstborn male will be set apart to the Lord"),
Hebrews 12:23
and congregation of the firstborn, who are enrolled in heaven, and to God, the judge of all, and to the spirits of the righteous, who have been made perfect,

On the day

Exodus 12:29
It happened at midnight -- the Lord attacked all the firstborn in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the captive who was in the prison, and all the firstborn of the cattle.
Exodus 13:15
When Pharaoh stubbornly refused to release us, the Lord killed all the firstborn in the land of Egypt, from the firstborn of people to the firstborn of animals. That is why I am sacrificing to the Lord the first male offspring of every womb, but all my firstborn sons I redeem.'

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