Numbers 3:13

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

Exodus 13:2

Sanctify unto me all the firstborn, whatsoever opens the womb among the children of Israel, both of man and of beast: it is mine.

Exodus 13:12

That you shall set apart unto the LORD all that opens the womb, and every firstling that comes of a beast which you have; the males shall be the LORD'S.

Exodus 13:15

And it came to pass, when Pharaoh refused to let us go, that the LORD slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the LORD all that opens the womb, being males; but all the firstborn of my children I redeem.

Leviticus 27:26

Only the firstling of the animals, which should be the LORD'S firstling, no man may dedicate it; whether it be ox, or sheep: it is the LORD'S.

Exodus 12:29-30

And it came to pass, that at midnight the LORD smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne to the firstborn of the captive that was in the dungeon; and all the firstborn of cattle.

Exodus 22:29

You shall not delay to offer the first of your ripe fruits, and of the outflow of your presses: the firstborn of your sons shall you give unto me.

Exodus 34:19

All that opens the womb is mine; and every firstling among your cattle, whether ox or sheep, that is male.

Numbers 8:16-17

For they are wholly given unto me from among the children of Israel; instead of all that open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.

Numbers 18:15

Every thing that opens the womb in all flesh, which they bring unto the LORD, whether it be of men or animals, shall be yours: nevertheless the firstborn of man shall you surely redeem, and the firstling of unclean animals shall you redeem.

Ezekiel 44:30

And the first of all the firstfruits of all things, and every offering of all, of every sort of your offerings, shall be the priest's: you shall also give unto the priest the first of your dough, that he may cause the blessing to rest in your house.

Luke 2:23

(As it is written in the law of the Lord, Every 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 written in heaven, and to God the Judge of all, and to the spirits of just men 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 unto me from among the children of Israel; instead of all that open every womb, even instead of the firstborn of all the children of Israel, have I taken them unto me.
Numbers 18:15
Every thing that opens the womb in all flesh, which they bring unto the LORD, whether it be of men or animals, shall be yours: nevertheless the firstborn of man shall you surely redeem, and the firstling of unclean animals shall you redeem.
Exodus 13:2
Sanctify unto me all the firstborn, whatsoever opens the womb among the children of Israel, both of man and of beast: it is mine.
Exodus 22:29
You shall not delay to offer the first of your ripe fruits, and of the outflow of your presses: the firstborn of your sons shall you give unto me.
Exodus 34:19
All that opens the womb is mine; and every firstling among your cattle, whether ox or sheep, that is male.
Leviticus 27:26
Only the firstling of the animals, which should be the LORD'S firstling, no man may dedicate it; whether it be ox, or sheep: it is the LORD'S.
Ezekiel 44:30
And the first of all the firstfruits of all things, and every offering of all, of every sort of your offerings, shall be the priest's: you shall also give unto the priest the first of your dough, that he may cause the blessing to rest in your house.
Luke 2:23
(As it is written in the law of the Lord, Every 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 written in heaven, and to God the Judge of all, and to the spirits of just men made perfect,

On the day

Exodus 12:29
And it came to pass, that at midnight the LORD smote all the firstborn in the land of Egypt, from the firstborn of Pharaoh that sat on his throne to the firstborn of the captive that was in the dungeon; and all the firstborn of cattle.
Exodus 13:15
And it came to pass, when Pharaoh refused to let us go, that the LORD slew all the firstborn in the land of Egypt, both the firstborn of man, and the firstborn of beast: therefore I sacrifice to the LORD all that opens the womb, being males; but all the firstborn of my children I redeem.