Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 244
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 244, with English translation by Maimonides (Rambam).
לדון בדין השואל, שנאמר: "וכי ישאל איש מעם ריעהו" (שמות כב,יג).
To render judgment [in questions] regarding a borrower, as [Exodus 22:13] states: "If a person will borrow from a colleague...."