Albert Barnes Commentary Numbers 3:13

"for all the first-born are mine; on the day that I smote all the first-born in the land of Egypt I hallowed unto me all the first-born in Israel, both man and beast; mine they shall be: I am Jehovah." — Numbers 3:13 (ASV)

The concluding words are better expressed thus: Mine shall they be, Mine, the Lord’s. On the subject of the firstborn, see the notes at (Numbers 3:43–51).