Negative commandment · Lo Ta’aseh #150 of 613
To not eat the second tithe in impurity
שלא לאכול מעשר שני בטומאה ואפילו בירושלים עד שייפדה
The verse · Devarim 26:14
לֹא־אָכַלְתִּי בְאֹנִי מִמֶּנּוּ וְלֹא־בִעַרְתִּי מִמֶּנּוּ בְּטָמֵא וְלֹא־נָתַתִּי מִמֶּנּוּ לְמֵת שָׁמַעְתִּי בְּקוֹל ה׳ אֱלֹהָי עָשִׂיתִי כְּכֹל אֲשֶׁר צִוִּיתָנִי
“I have not eaten of it in my mourning, neither have I consumed any part of it when unclean, nor given of it for the dead: but I have hearkened to the voice of the Lord my God, and have done according to all that thou hast commanded me.”
The Rambam’s words
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."
שלא לאכול מעשר שני בטומאה ואפילו בירושלים עד שייפדה, שנאמר "לא ביערתי ממנו בטמא" (דברים כו,יד).
Why · Sefer HaChinuch #609
To not eat the second tithe in impurity
“It is from the roots of the commandment [that it is] because it is known to the Sages that the matter of impurity weakens the power of the intellect and mixes it up and separates between it and the pure and perfect Elevated Understanding.”
Who & when
“And this prohibition is practiced by males and females at the time of the [Temple], as we had tithes from Torah writ [then].”
Where in the Mishneh Torah
Hilchot Second Tithes and Fourth Year's Fruit הלכות מעשר שני ונטע רבעי