Positive commandment · Mitzvat Aseh #204 of 613
To return a lost object
להשיב האבידה
The verse · Devarim 22:1
לֹא־תִרְאֶה אֶת־שׁוֹר אָחִיךָ אוֹ אֶת־שֵׂיוֹ נִדָּחִים וְהִתְעַלַּמְתָּ מֵהֶם הָשֵׁב תְּשִׁיבֵם לְאָחִיךָ
“Thou shalt not see thy brother’s ox or his sheep go astray, and hide thyself from them: thou shalt surely bring them back to thy brother.”
The Rambam’s words
To return a lost object, as [Deuteronomy 22:1] states: "You shall surely return them to your brother."
להשיב האבידה, שנאמר: "השב תשיבם, לאחיך" (דברים כב,א).
Why · Sefer HaChinuch #538
To return a lost item to an Israelite
“The root of this commandment is known — as there is in it a benefit to all, and to the ordering of the state; as forgetfulness exists with all [people, and] also their beasts and all of their animals flee here and there.”
Who & when
“And it is practiced in every place and at all times, by males and females.”
Where in the Mishneh Torah
Hilchot Robbery and Lost Property הלכות גזילה ואבידה