Jerusalem Talmud Bava Metzia 2.7

משנה: אָמַר אֶת הָאֲבֵידָה וְלֹא אָמַר אֶת סֵימָנֶיהָ הֲרֵי זֶה לֹא יִתֵּן. וְהָרַמַּאי אַף עַל פִּי שֶׁאָמַר סֵימָנֶיהָ הֲרֵי זֶה לֹא יִתֵּן לוֹ שֶׁנֶּאֱמַר עַד דְּרוֹשׁ אָחִיךָ אוֹתוֹ. עַד שֶׁתִּדְרוֹשׁ אֶת אָחִיךָ אִם רַמַּאי הוּא אִם אֵינוֹ רַמַּאי.

MISHNAH: If one described the lost object but not its identifying marks, he should not deliver. To the swindler one should not deliver even if he describes its identifying marks, since it is said: “Until investigating your brother about it,” until you investigate your brother whether he be a swindler or no swindler.

הלכה: אָמַר אֶת הָאֲבֵידָה כול׳. אֵי זֶהוּ רַמַּאי. עֲבַד גַּרְמֵיהּ מְחַזֵּר מְצִיאָן. וַעֲבַד חָדָא רַבָּה וְטָבָה. הֵיךְ הוּא עֲבִיד רַבָּה וְטָבָא. חֲמֵי לֵיהּ בִּירְיָיתָא מְחַזֵּר מְצִיאָן וּמַפְקִידִין גַּבֵּיהּ וְהוּא נְסִיב כּוּלָּאּ וַאֲזִיל לֵיהּ. וְאִית דָּמְרִין. אֲזִיל לִכְנִישְׁתָּא וּשְׁמַע סֵימָנִין וַאֲזַל לִכְנִישְׁתָּא חוֹרֵי וַאֲמַר סֵימָנָא וּנְסַב לֵיהּ.

HALAKHAH: “If one described the lost object,” etc. What is a swindler? He shows himself as one who returns finds and uses this for a big coup. How does he make a big coup? He shows himself to people as one who returns finds so they will give him valuables in trust. He takes all of them and disappears. But some say, he goes to one synagogue and hears about distinguishing marks; then he goes to another synagogue, describes the indentifying marks and takes [the finds] away.