Positive commandment · Mitzvat Aseh #128 of 613
To separate the second tithe so that it can be eaten by its owners in Jerusalem
להפריש מעשר שני להיאכל לבעליו בירושלים
The verse · Devarim 14:22
עַשֵּׂר תְּעַשֵּׂר אֵת כׇּל־תְּבוּאַת זַרְעֶךָ הַיֹּצֵא הַשָּׂדֶה שָׁנָה שָׁנָה
“Thou shalt truly tithe all the increase of thy seed, that the field brings forth year by year.”
The Rambam’s words
To separate the second tithe so that it can be eaten by its owners in Jerusalem, as [Deuteronomy 14:22] states: "You shall surely tithe...." According to the oral tradition, we learn that this refers to the second tithe.
להפריש מעשר שני להיאכל לבעליו בירושלים, שנאמר: "עַשֵּׂר תְּעַשֵּׂר" (דברים יד, כב) מפי השמועה למדו שזה הוא מעשר שני.
Why · Sefer HaChinuch #473
Second tithe
“It is from the roots of the commandment [that it is because] G-d chose the nation of Israel and wished for the sake of His righteousness that all of them be engaged with His Torah and be knowers of His Name.”
Who & when
“And this commandment is only practiced anywhere at the time that Israel is dwelling on their Land and Jerusalem is inhabited — may we merit that our eyes see its goodness.”
Where in the Mishneh Torah
Hilchot Second Tithes and Fourth Year's Fruit הלכות מעשר שני ונטע רבעי