Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 241
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 241, with English translation by Maimonides (Rambam).
שלא למשכן האלמנה, שנאמר "ולא תחבול בגד אלמנה" (דברים כד, יז).
Not to take security from a widow, as [Deuteronomy 24:17] states: "Do not take a widow's garment as security."