Bava Metzia, Chapter 2, Mishnah 5. This Mishnah returns to discuss the basic source from which we learn that there is an obligation to announce a lost item that has identifying marks. To do this, the Mishnah goes back and refers to the verses we saw at the beginning of the chapter, and we will preface by reading them again.
The Verses at the Foundation of the Topic (Devarim 22):
"Lo tireh es shor achicha o es seyo nidachim vehisalamta meihem, hashev teshivem leachicha" - One may not see his brother's ox, sheep, or goat wandering lost and turn a blind eye to them. This verse includes the prohibition against ignoring a lost item and the positive commandment of returning it.
The second verse states: "Ve'im lo karov achicha eilecha velo yedasto, va'asafto el toch beisecha vehaya imcha ad derosh achicha oto vahashevoso lo" - When the finder does not know to whom the lost item belongs, or the owner is not around, he must safeguard it until his brother comes and claims it.
And the third verse: "Vechein ta'aseh lachamoro vechein ta'aseh lesimlaso, vechein ta'aseh lechol avedas achicha asher tovad mimenu umetzasah, lo tuchal lehisalem" - After stating the law regarding an ox and a sheep, the Torah adds the donkey and the garment, establishing that the same law applies to any lost item that your brother loses and the finder finds, and he is not permitted to turn a blind eye to it.
From here the question arises: Why did the Torah specify the donkey and the garment, when it already states "and so shall you do for every lost item of your brother" - meaning any lost item whatsoever? This is the starting point of our Mishnah.
The Text of the Mishnah:
The Mishnah says: "Af hasimlah hayesa bichlal kol eileh, velamah yatzas? Lehakesh eileha lomar lach: mah simlah meyuchedes sheyesh bah simanim veyesh lah tove'im" - The garment was included in the general category of "every lost item of your brother", and if it nevertheless was singled out and specified on its own, it is in order to make a juxtaposition (hekesh) and learn from it regarding all other lost items. A garment has two unique characteristics:
"Sheyesh bah simanim" - A garment in the time of the Mishnah was handmade, and therefore each person's garment was unique to him: a different size, a different shape, a different style of sewing, a unique placement of buttons, and the like. Consequently, it always has identifying marks.
"Veyesh lah tove'im" - A garment does not grow from the ground, and there are no garments in the world wandering around ownerless. Every garment belonged to someone, and there is certainly someone who lost it and wants to get it back.
The garment is therefore an archetype: a model of a lost object that has identifying marks and has someone claiming it. By its power the juxtaposition is made: "Af kol davar sheyesh bo simanim veyesh lo tove'im, chayav lehachriz" - For any object in the world that has unique characteristics and has someone who lost it and wants it back, the finder is obligated to announce it and make known that he found it.
The Rishonim explain that from here arises the Torah-level obligation of the announcement - to announce the finding and search for the one who lost it. This obligation is learned from this source, from the very derivation of the word "simlaso" (his garment).
The Additional Inclusions in the Verse:
"Chamor" - The mention of the donkey comes for a separate derivation, from which we learn that if the person who lost it cannot identify the donkey itself but can identify the saddle on it, this is a sufficient identifying mark to receive back not only the saddle, but even the donkey attached to it.
"Shor" - The Gemara explains that this inclusion comes to teach that even the shorn hair at the tip of the ox's tail, which has no real value, is also included in the obligation to return lost property.
"Seh" - As for the mention of a person's seh - the goat or the sheep - it certainly comes to include something, but the Gemara ultimately leaves this as an unresolved question.