Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 142
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 142, with English translation by Maimonides (Rambam).
לנגוש לנוכרי, שנאמר: "את הנוכרי, תיגוש; ואשר יהיה לך" (דברים טו,ג).
To seek to collect [a debt] from a gentile [in the seventh year], as [Deuteronomy 15:3] states: "Seek to collect [a debt] from a gentile. However, what your brother owes you must remit...."