Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 242
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 242, with English translation by Maimonides (Rambam).
לדון בדין שומר חינם, שנאמר: "כי ייתן איש אל ריעהו כסף או כלים" (שמות כב,ו).
To render judgment [in questions] regarding an unpaid watchman, as [Exodus 22:6] states: "If a person will give a colleague money or utensils [to watch]...."