Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 111
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 111, with English translation by Maimonides (Rambam).
שלא לפדות שדה החרם, שנאמר "לא ייגאל" (ויקרא כז,כח).
Not to redeem a field that has been dedicated, as [Leviticus, ibid.] states: "It shall not be redeemed."