Numbers 3:46-51
And for those that are to be redeemed of the two hundred and three score and thirteen of the firstborn of the children of Israel, which are more than the Levites;
Nave's Topical Index—AKJV

Numbers 18:15,16
Every thing that opens the matrix in all flesh, which they bring to the LORD, whether it be of men or beasts, shall be yours: nevertheless the firstborn of man shall you surely redeem, and the firstling of unclean beasts shall you redeem.
Nave's Topical Index—AKJV