Numbers 3:13

Because all the firstborn [are] mine
Not merely in a general way, as all creatures are his, but in a special manner as his own, and that for the following reason:

[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;
that is, sanctified or set them apart as his own special property, or ordered the people of Israel so to do, ( Exodus 13:2 ) ; for as when he destroyed the firstborn of the Egyptians, he saved the firstborn of Israel, he had a special claim upon them as his; and though it was in the night when he destroyed the firstborn of Egypt, yet it was the night which preceded that day, and was a part of that day, even the fifteenth of Nisan, when the instructions were given to sanctify all the firstborn; though, as Aben Ezra observes, "day" signifies "time", so that it was at or about the same time that the one and the other were done:

mine they shall be;
this was declared when they were ordered to be sanctified to him, but now they were to be exchanged for the Levites:

I [am] the Lord;
who have sovereign power to do as he would in claiming the firstborn, and then in exchanging them for the Levites, and appointing the Levites to minister to the priests, and serve in the tabernacle.