Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 217
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 217, with English translation by Maimonides (Rambam).
לחלוץ ליבם, שנאמר: "וחלצה נעלו מעל רגלו" (דברים כה,ט).
To remove the yavam's shoe [if he does not marry his brother's widow], as [Deuteronomy 25:9] states: "And she shall remove his shoe from his foot."