Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 139
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 139, with English translation by Maimonides (Rambam).
להיות גאולה בבתי ערי חומה עד שנה, שנאמר: "ואיש, כי ימכור בית מושב" (ויקרא כה,כט).
To allow houses in a walled city to be redeemed within a year, as [Leviticus 25:29] states: "If a person shall sell a residential house in a walled city...."