Pidyon Haben פדיון הבן

The firstborn have belonged to God since the night the firstborn of Egypt died and those of Israel lived. Their service passed to the Levites after the sin of the calf—but the belonging itself remained, and a father redeems his son from a priest for five shekels of silver, as the Torah fixed.

The rite is simple and ancient: the priest asks the father which he prefers—his son or the five coins; the father answers that he prefers his son, pays, and says the blessing. Only the mother's firstborn is redeemed, and not the son of a priest or a Levite. The details close tractate Bekhorot.