Numbers 8:17

For all the firstborn of the children of Israel [are] mine,
[both] men and beasts
Not only in common with other of his creatures, but in a special respect on account of his signal deliverance of them; they were his, as Jarchi expresses it, by the line of judgment, or rule of justice and equity, because he protected them among the firstborn of the Egyptians, saved them when he slew theirs, as follows:

in the day that I smote every firstborn in the land of Egypt;
both of men and beasts:

I sanctified them for myself;
claimed them as his own, and set them apart for his own use, or ordered them to be set apart: see ( Exodus 13:2 ) .