Numbers 18:16

Overview - Numbers 18
The charge of the priests and Levites.
The priests' portion.
21 The Levites' portion.
25 The heave offering to the priests out of the Levites' portion.
Treasury of Scripture Knowledge

Numbers 18:16  (King James Version)
And those that are to be redeemed from a month old shalt thou redeem, according to thine estimation, for the money of five shekels, after the shekel of the sanctuary, which is twenty gerahs.

shalt thou redeem
Redemption of the firstborn is one of the rites which are still practised among the Jews. According to Leo of Modena, it is performed in the following manner:--When the child is thirty days old, the father sends for one of the descendants of Aaron: several persons being assembled on the occasion, the father brings a cup, containing several pieces of gold and silver coin. The priest then takes the child into his arms, and addressing himself to the mother, he says, "Is this thy son?" Mother
"Yes." Priest. "Hast thou never had another child, male or female, a miscarriage, or untimely birth?" Mother
"No." Priest. "This being the case, this child, as firstborn, belongs to me." Then turning to the father, he says, "If it be thy desire to have this child, thou must redeem it
" Father. "I present thee with this gold and silver for this purpose
" Priest. "Thou dost wish, therefore, to redeem this child?" Father. "I do wish so to do." The priest then turning himself to the assembly, says, "Very well: this child, as first-born, is mine; as it is
written in Bemidbar, (ch. 18:16, ) Thou shalt redeem the first-born of a month old for five shekels; but I shall content myself with this in exchange." He then takes two gold crowns, or thereabouts, and returns the child to his parents.* according.
3:47 Le 27:2-7

which is
Exodus 30:13 ; Leviticus 27:25 ; Ezekiel 45:12