Numbers 3:13

because every firstborn belongs to Me. At the time I struck down every firstborn in the land of Egypt, I consecrated every firstborn in Israel to Myself, both man and animal. They are Mine; I am Yahweh.”

Exodus 13:2

“Consecrate every firstborn male to Me, the firstborn from every womb among the Israelites, both man and domestic animal; it is Mine.”

Exodus 13:12

you are to present to the Lord every firstborn male of the womb. All firstborn offspring of the livestock you own that are males will be the Lord’s.

Exodus 13:15

When Pharaoh stubbornly refused to let us go, the Lord killed every firstborn male in the land of Egypt, from the firstborn of man to the firstborn of livestock. That is why I sacrifice to the Lord all the firstborn of the womb that are males, but I redeem all the firstborn of my sons.’

Leviticus 27:26

“But no one can consecrate a firstborn of the livestock, whether an animal from the herd or flock, to the Lord, because a firstborn already belongs to the Lord.

Exodus 12:29-30

Now at midnight the Lord struck every firstborn male in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the prisoner who was in the dungeon, and every firstborn of the livestock.

Exodus 22:29

“You must not hold back offerings from your harvest or your vats. Give Me the firstborn of your sons.

Exodus 34:19

“The firstborn male from every womb belongs to Me, including all your male livestock, the firstborn of cattle or sheep.

Numbers 8:16-17

For they have been exclusively assigned to Me from the Israelites. I have taken them for Myself in place of all who come first from the womb, every Israelite firstborn.

Numbers 18:15

The firstborn of every living thing, man or animal, presented to the Lord belongs to you. But you must certainly redeem the firstborn of man, and redeem the firstborn of an unclean animal.

Ezekiel 44:30

The best of all the firstfruits of every kind and contribution of every kind from all your gifts will belong to the priests. You are to give your first batch of dough to the priest so that a blessing may rest on your homes.

Luke 2:23

(just as it is written in the law of the Lord: Every firstborn male will be dedicated to the Lord)

Hebrews 12:23

to the assembly of the firstborn whose names have been written in heaven, to God who is the Judge of all, to the spirits of righteous people 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 have been exclusively assigned to Me from the Israelites. I have taken them for Myself in place of all who come first from the womb, every Israelite firstborn.
Numbers 18:15
The firstborn of every living thing, man or animal, presented to the Lord belongs to you. But you must certainly redeem the firstborn of man, and redeem the firstborn of an unclean animal.
Exodus 13:2
“Consecrate every firstborn male to Me, the firstborn from every womb among the Israelites, both man and domestic animal; it is Mine.”
Exodus 22:29
“You must not hold back offerings from your harvest or your vats. Give Me the firstborn of your sons.
Exodus 34:19
“The firstborn male from every womb belongs to Me, including all your male livestock, the firstborn of cattle or sheep.
Leviticus 27:26
“But no one can consecrate a firstborn of the livestock, whether an animal from the herd or flock, to the Lord, because a firstborn already belongs to the Lord.
Ezekiel 44:30
The best of all the firstfruits of every kind and contribution of every kind from all your gifts will belong to the priests. You are to give your first batch of dough to the priest so that a blessing may rest on your homes.
Luke 2:23
(just as it is written in the law of the Lord: Every firstborn male will be dedicated to the Lord)
Hebrews 12:23
to the assembly of the firstborn whose names have been written in heaven, to God who is the Judge of all, to the spirits of righteous people made perfect,

On the day

Exodus 12:29
Now at midnight the Lord struck every firstborn male in the land of Egypt, from the firstborn of Pharaoh who sat on his throne to the firstborn of the prisoner who was in the dungeon, and every firstborn of the livestock.
Exodus 13:15
When Pharaoh stubbornly refused to let us go, the Lord killed every firstborn male in the land of Egypt, from the firstborn of man to the firstborn of livestock. That is why I sacrifice to the Lord all the firstborn of the womb that are males, but I redeem all the firstborn of my sons.’

Holman Christian Standard Bible®, Copyright © 1999, 2000, 2002, 2003, 2009 by Holman Bible Publishers.