Parashas Bamidbar · Passuk 3:48
and give the money to Aaron and his sons as the redemption price for those who are in excess.
Chabad.org — English translation + Rashi
Source: Bechoros 5a.
English: “And you shall give the money to Aharon and his sons” — the redemption of the [human] firstborn is given to the males of the priesthood.
In context: The verse is the source that pidyon ha-ben money belongs to the kohanim.
Custom
Chazal derive that the five-shekel redemption payment is given to male kohanim.