Numbers 18:15 "Nevertheless the first-born of man you must surely redeem"

Since our first-born sons were claimed by HaShem, they must be redeemed by giving five silver coins to the priest.

Fathers should fulfill this Positive Mitzvah when the baby boy is one month old. This is called the Pidyon HaBen.

If for some reason the Pidyon HaBen did not take place at that time, the first-born son must carry out the Pidyon HaBen himself.