Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 198
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 198, with English translation by Maimonides (Rambam).
להלוות לנוכרי בריבית, שנאמר: "לנוכרי תשיך" (דברים כג,כא); מפי השמועה למדו שזו מצות עשה.
To lend to a gentile at interest, as [Deuteronomy 23:21] states: "Take interest from a gentile." Based on the oral tradition, we have learned that this is a positive commandment.