Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 109
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 109, with English translation by Maimonides (Rambam).
שלא למכור מעשר בהמה, שנאמר "לא ייגאל" (ויקרא כז,לג).
Not to sell the tithe of cattle, as [Leviticus 27:33] states: "It shall not be redeemed."