Numbers 3:13

For all the firstborn are Mine; on the day that I struck down all the firstborn in the land of Egypt, I sanctified to Myself all the firstborn in Israel, from man to beast. They shall be Mine; I am the Lord.”

Exodus 13:2

Sanctify to Me every firstborn, the first offspring of every womb among the sons of Israel, both of man and beast; it belongs to Me.”

Exodus 13:12

you shall devote to the Lord the first offspring of every womb, and the first offspring of every beast that you own; the males belong to the Lord.

Exodus 13:15

It came about, when Pharaoh was stubborn about letting us go, that the Lord killed every firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore, I sacrifice to the Lord the males, the first offspring of every womb, but every firstborn of my sons I redeem.’

Leviticus 27:26

However, a firstborn among animals, which as a firstborn belongs to the Lord, no man may consecrate it; whether ox or sheep, it is the Lord’s.

Exodus 12:29-30

Now it came about at midnight that the Lord struck 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 dungeon, and all the firstborn of cattle.

Exodus 22:29

You shall not delay the offering from your harvest and your vintage. The firstborn of your sons you shall give to Me.

Exodus 34:19

The first offspring from every womb belongs to Me, and all your male livestock, the first offspring from cattle and sheep.

Numbers 8:16-17

for they are wholly given to Me from among the sons of Israel. I have taken them for Myself instead of every first issue of the womb, the firstborn of all the sons of Israel.

Numbers 18:15

Every first issue of the womb of all flesh, whether man or animal, which they offer to the Lord, shall be yours; nevertheless the firstborn of man you shall surely redeem, and the firstborn of unclean animals you shall redeem.

Ezekiel 44:30

The first of all the first fruits of every kind and every contribution of every kind, from all your contributions, shall be for the priests; you shall also give to the priest the first of your dough to cause a blessing to rest on your house.

Luke 2:23

(as it is written in the Law of the Lord, “Every firstborn male that opens the womb shall be called holy to the Lord”),

Hebrews 12:23

to the general assembly and church of the firstborn who are enrolled in heaven, and to God, the Judge of all, and to the spirits of the righteous 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 wholly given to Me from among the sons of Israel. I have taken them for Myself instead of every first issue of the womb, the firstborn of all the sons of Israel.
Numbers 18:15
Every first issue of the womb of all flesh, whether man or animal, which they offer to the Lord, shall be yours; nevertheless the firstborn of man you shall surely redeem, and the firstborn of unclean animals you shall redeem.
Exodus 13:2
Sanctify to Me every firstborn, the first offspring of every womb among the sons of Israel, both of man and beast; it belongs to Me.”
Exodus 22:29
You shall not delay the offering from your harvest and your vintage. The firstborn of your sons you shall give to Me.
Exodus 34:19
The first offspring from every womb belongs to Me, and all your male livestock, the first offspring from cattle and sheep.
Leviticus 27:26
However, a firstborn among animals, which as a firstborn belongs to the Lord, no man may consecrate it; whether ox or sheep, it is the Lord’s.
Ezekiel 44:30
The first of all the first fruits of every kind and every contribution of every kind, from all your contributions, shall be for the priests; you shall also give to the priest the first of your dough to cause a blessing to rest on your house.
Luke 2:23
(as it is written in the Law of the Lord, “Every firstborn male that opens the womb shall be called holy to the Lord”),
Hebrews 12:23
to the general assembly and church of the firstborn who are enrolled in heaven, and to God, the Judge of all, and to the spirits of the righteous made perfect,

On the day

Exodus 12:29
Now it came about at midnight that the Lord struck 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 dungeon, and all the firstborn of cattle.
Exodus 13:15
It came about, when Pharaoh was stubborn about letting us go, that the Lord killed every firstborn in the land of Egypt, both the firstborn of man and the firstborn of beast. Therefore, I sacrifice to the Lord the males, the first offspring of every womb, but every firstborn of my sons I redeem.’