Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 236
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 236, with English translation by Maimonides (Rambam).
שלא ללוות בריבית, שנאמר "לא תשיך לאחיך" (דברים כג,כ); כך למדו מפי השמועה, שזו אזהרה ללווה שלא יינשך למלווה.
Not to borrow with interest, as [inferred from Deuteronomy 23:20, which] states: "Do not take interest from your brother." According to the oral tradition, this verse is interpreted as a prohibition, forbidding a borrower from paying interest to a lender.