Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 150
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 150, with English translation by Maimonides (Rambam).
שלא לאכול מעשר שני בטומאה ואפילו בירושלים עד שייפדה, שנאמר "לא ביערתי ממנו בטמא" (דברים כו,יד).
Not to eat the second tithe which has become impure, even within Jerusalem, until it has been redeemed, as [Deuteronomy 26:14] states: "I have not consumed it while it is impure."