Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 204
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 204, with English translation by Maimonides (Rambam).
להשיב האבידה, שנאמר: "השב תשיבם, לאחיך" (דברים כב,א).
To return a lost object, as [Deuteronomy 22:1] states: "You shall surely return them to your brother."