Numbers 3:13

for all the firstborn are Mine. On the day that I killed all the firstborn in the land of Egypt, I consecrated for Myself all the firstborn in Israel, both man and animal. They shall be mine; I am the Lord.”

Exodus 13:2

“Sanctify to Me [that is, set apart for My purpose] every firstborn, the first offspring of every womb among the children of Israel, both of man and of animal; it is Mine.”

Exodus 13:12

you shall set apart and dedicate to the Lord all that first opens the womb. All the firstborn males of your livestock shall be the Lord’s.

Exodus 13:15

For it happened, when Pharaoh stubbornly refused to let us go, that the Lord struck every firstborn in the land of Egypt, both the firstborn of man and the firstborn of animal. Therefore, I sacrifice to the Lord all the males, the first [to be born] of every womb, but every firstborn of my sons I redeem.’

Leviticus 27:26

‘However, the firstborn among animals, which as a firstborn belongs to the Lord, no man may consecrate, whether an ox or a sheep. It is [already] the Lord’s.

Exodus 12:29-30

Now it happened at midnight that the Lord struck every firstborn 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 all the firstborn of the cattle.

Exodus 22:29

“You shall not delay the offering from your harvest and your vintage. You shall give (consecrate, dedicate) to Me the firstborn of your sons.

Exodus 34:19

“All the firstborn males among your livestock belong to Me, whether cattle or sheep.

Numbers 8:16-17

for they are wholly given to Me from among the Israelites. I have taken them for Myself instead of all who are born first, the firstborn of all the Israelites.

Numbers 18:15

Every firstborn of the womb of all flesh, whether it is man or animal, which they bring to the Lord, shall be yours; nevertheless, the firstborn of man you shall most certainly 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 and offering of every kind, from all your contributions and offerings, shall belong to the priests. You shall also give to the priest the first of your coarse meal and bread dough, so that a blessing may 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 [set apart and dedicated] to the Lord)”

Hebrews 12:23

and to the general assembly and assembly of the firstborn who are registered [as citizens] in heaven, and to God, who is Judge of all, and to the spirits of the righteous (the redeemed in heaven) who have been made perfect [bringing them to their final glory],

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 Israelites. I have taken them for Myself instead of all who are born first, the firstborn of all the Israelites.
Numbers 18:15
Every firstborn of the womb of all flesh, whether it is man or animal, which they bring to the Lord, shall be yours; nevertheless, the firstborn of man you shall most certainly redeem, and the firstborn of unclean animals you shall redeem.
Exodus 13:2
“Sanctify to Me [that is, set apart for My purpose] every firstborn, the first offspring of every womb among the children of Israel, both of man and of animal; it is Mine.”
Exodus 22:29
“You shall not delay the offering from your harvest and your vintage. You shall give (consecrate, dedicate) to Me the firstborn of your sons.
Exodus 34:19
“All the firstborn males among your livestock belong to Me, whether cattle or sheep.
Leviticus 27:26
‘However, the firstborn among animals, which as a firstborn belongs to the Lord, no man may consecrate, whether an ox or a sheep. It is [already] the Lord’s.
Ezekiel 44:30
The first of all the first fruits of every kind, and every contribution and offering of every kind, from all your contributions and offerings, shall belong to the priests. You shall also give to the priest the first of your coarse meal and bread dough, so that a blessing may 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 [set apart and dedicated] to the Lord)”
Hebrews 12:23
and to the general assembly and assembly of the firstborn who are registered [as citizens] in heaven, and to God, who is Judge of all, and to the spirits of the righteous (the redeemed in heaven) who have been made perfect [bringing them to their final glory],

On the day

Exodus 12:29
Now it happened at midnight that the Lord struck every firstborn 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 all the firstborn of the cattle.
Exodus 13:15
For it happened, when Pharaoh stubbornly refused to let us go, that the Lord struck every firstborn in the land of Egypt, both the firstborn of man and the firstborn of animal. Therefore, I sacrifice to the Lord all the males, the first [to be born] of every womb, but every firstborn of my sons I redeem.’