Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 128
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 128, with English translation by Maimonides (Rambam).
להפריש מעשר שני להיאכל לבעליו בירושלים, שנאמר: "עַשֵּׂר תְּעַשֵּׂר" (דברים יד, כב) מפי השמועה למדו שזה הוא מעשר שני.
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.