Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 80
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 80, with English translation by Maimonides (Rambam).
לפדות בכור אדם, שנאמר: "אך פדה תפדה, את בכור האדם" (במדבר יח,טו).
To redeem firstborn sons, as [Numbers 18:15] states: "However, you must surely redeem first-born humans."