Numbers 3:13

for, mine, is every firstborn, in the day when I smote every firstborn in the land of Egypt, hallowed unto myself every firstborn in Israel, both of man and of beast, - mine, did they become, and belong unto me, Yahweh.

Exodus 13:2

Hallow to me every firstborn, that which any female first beareth among the sons of Israel, among men and among beasts, - mine! it is.

Exodus 13:12

that thou shalt set apart whatsoever is born first, to Yahweh; and, of all firstlings of beasts which thou shalt have, the males, shall pertain to Yahweh.

Exodus 13:15

And it came to pass when Pharaoh had shown himself too hardened to let us go, then did Yahweh slay every firstborn in the land of Egypt, from the firstborn of men even unto the firstborn of beasts. For this cause, am, I, sacrificing to Yahweh whatsoever is born first, of the males, and every firstborn of my sons, must I redeem.

Leviticus 27:26

Only the firstling which is born a firstling to Yahweh, among beasts, no man shall hallow it, - whether ox or sheep, unto Yahweh, it belongeth.

Exodus 12:29-30

And it came to pass, at midnight, that Yahweh, smote every firstborn in the land of Egypt, from the firstborn of Pharaoh who sat upon his throne, unto the firstborn of the captive who was in the pit of his prison, - and every firstborn of beasts.

Exodus 22:29

Thy fulness and thine outflow, shalt thou not delay, - the firstborn of thy sons, shalt thou give to me.

Exodus 34:19

Every one that is the first to be born, is mine, - and every one of thy cattle that is born a male, firstling of oxen or sheep.

Numbers 8:16-17

For, given, given, they are unto me, out of the midst of the sons of Israel, - instead of every firstborn that a mother beareth from among the sons of Israel, have I taken them unto me.

Numbers 18:15

every thing that is born first of all flesh that may be offered unto Yahweh among men and among beasts, shall be thine, - only thou shalt, redeem, the firstborn of men, and the firstborn of unclean beasts, shalt thou redeem.

Ezekiel 44:30

And the first of all the firstfruits of everything, and every heave-offering of everything, from among all your heave-offerings to the priests, shall it belong, - The first of your meal, shall ye give to the priest, To cause a blessing to rest upon by house.

Luke 2:23

according as it is written in the law of the Lord - Every male that is a firstborn, Holy unto the Lord, shall be called,

Hebrews 12:23

in high festival, - and unto an assembly of firstborn ones, enrolled in the heavens, - and unto God, judge of all, - and unto the spirits of righteous ones 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, given, given, they are unto me, out of the midst of the sons of Israel, - instead of every firstborn that a mother beareth from among the sons of Israel, have I taken them unto me.
Numbers 18:15
every thing that is born first of all flesh that may be offered unto Yahweh among men and among beasts, shall be thine, - only thou shalt, redeem, the firstborn of men, and the firstborn of unclean beasts, shalt thou redeem.
Exodus 13:2
Hallow to me every firstborn, that which any female first beareth among the sons of Israel, among men and among beasts, - mine! it is.
Exodus 22:29
Thy fulness and thine outflow, shalt thou not delay, - the firstborn of thy sons, shalt thou give to me.
Exodus 34:19
Every one that is the first to be born, is mine, - and every one of thy cattle that is born a male, firstling of oxen or sheep.
Leviticus 27:26
Only the firstling which is born a firstling to Yahweh, among beasts, no man shall hallow it, - whether ox or sheep, unto Yahweh, it belongeth.
Ezekiel 44:30
And the first of all the firstfruits of everything, and every heave-offering of everything, from among all your heave-offerings to the priests, shall it belong, - The first of your meal, shall ye give to the priest, To cause a blessing to rest upon by house.
Luke 2:23
according as it is written in the law of the Lord - Every male that is a firstborn, Holy unto the Lord, shall be called,
Hebrews 12:23
in high festival, - and unto an assembly of firstborn ones, enrolled in the heavens, - and unto God, judge of all, - and unto the spirits of righteous ones made perfect, -

On the day

Exodus 12:29
And it came to pass, at midnight, that Yahweh, smote every firstborn in the land of Egypt, from the firstborn of Pharaoh who sat upon his throne, unto the firstborn of the captive who was in the pit of his prison, - and every firstborn of beasts.
Exodus 13:15
And it came to pass when Pharaoh had shown himself too hardened to let us go, then did Yahweh slay every firstborn in the land of Egypt, from the firstborn of men even unto the firstborn of beasts. For this cause, am, I, sacrificing to Yahweh whatsoever is born first, of the males, and every firstborn of my sons, must I redeem.