"Redeem every firstborn of people." (Bamidbar 18: 15)
It is a mitzvah to redeem a firstborn son, through a kohein, for five
silver coins or their equivalent. This mitzvah is performed thirty days
after the baby's birth. If the father does not perform this mitzvah, the
firstborn himself is obligated in his redemption when he turns thirteen.
Why were we given this mitzvah? Hashem redeemed us from slavery with the
declaration: "My firstborn son is Yisrael" (Shemos 4:22). We redeem our
firstborn sons as a testimonial to this redemption, as it says, "Every
firstborn of your sons you shall redeem... because Hashem saved us from
Egypt with His great and mighty hand "(ibid. 13:13-14).