Numbers 3:13

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

Exodus 13:2

Sanctify to me all the first-born, whatever openeth the womb among the children of Israel, both of man and of beast: it is mine.

Exodus 13:12

That thou shall set apart to the LORD all that openeth the matrix; and every firstling that cometh of a beast which thou hast, the males shall be the LORD'S.

Exodus 13:15

And it came to pass, when Pharaoh would hardly let us go, that the LORD slew all the first-born in the land of Egypt, both the first-born of man, and the first-born of beasts: therefore I sacrifice to the LORD all that openeth the matrix, being males; but all the first-born of my children I redeem.

Leviticus 27:26

Only the firstling of the beasts, which should be the LORD'S firstling, no man shall sanctify it; whether 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 first-born in the land of Egypt, from the first-born of Pharaoh that sat on his throne, to the first-born of the captive that was in the dungeon; and all the first-born of cattle.

Exodus 22:29

Thou shalt not delay to offer the first of thy ripe fruits, and of thy liquors: the first-born of thy sons shalt thou give to me.

Exodus 34:19

Every first-born is mine: and every firstling among thy cattle, whether ox or sheep, that is male.

Numbers 8:16-17

For they are wholly given to me from among the children of Israel; instead of such as open every womb, even instead of the first-born of all the children of Israel, have I taken them to me.

Numbers 18:15

Every thing that openeth the matrix in all flesh, which they bring to the LORD, whether of men or beast, shall be thine: nevertheless, the first born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.

Ezekiel 44:30

And the first of all the first-fruits of all things, and every oblation of all, of every sort of your oblations, shall be the priest's: ye shall also give to the priest the first of your dough, that he may cause the blessing to rest in thy house.

Luke 2:23

(As it is written in the law of the Lord, Every male that is the first born of his mother shall be called holy to the Lord)

Hebrews 12:23

To the general assembly and church of the first-born, 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 to me from among the children of Israel; instead of such as open every womb, even instead of the first-born of all the children of Israel, have I taken them to me.
Numbers 18:15
Every thing that openeth the matrix in all flesh, which they bring to the LORD, whether of men or beast, shall be thine: nevertheless, the first born of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.
Exodus 13:2
Sanctify to me all the first-born, whatever openeth the womb among the children of Israel, both of man and of beast: it is mine.
Exodus 22:29
Thou shalt not delay to offer the first of thy ripe fruits, and of thy liquors: the first-born of thy sons shalt thou give to me.
Exodus 34:19
Every first-born is mine: and every firstling among thy cattle, whether ox or sheep, that is male.
Leviticus 27:26
Only the firstling of the beasts, which should be the LORD'S firstling, no man shall sanctify it; whether ox, or sheep: it is the LORD'S.
Ezekiel 44:30
And the first of all the first-fruits of all things, and every oblation of all, of every sort of your oblations, shall be the priest's: ye shall also give to the priest the first of your dough, that he may cause the blessing to rest in thy house.
Luke 2:23
(As it is written in the law of the Lord, Every male that is the first born of his mother shall be called holy to the Lord)
Hebrews 12:23
To the general assembly and church of the first-born, 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 first-born in the land of Egypt, from the first-born of Pharaoh that sat on his throne, to the first-born of the captive that was in the dungeon; and all the first-born of cattle.
Exodus 13:15
And it came to pass, when Pharaoh would hardly let us go, that the LORD slew all the first-born in the land of Egypt, both the first-born of man, and the first-born of beasts: therefore I sacrifice to the LORD all that openeth the matrix, being males; but all the first-born of my children I redeem.