Talmud for Demai 3:5

הַנּוֹתֵן לְפֻנְדָּקִית, מְעַשֵּׂר אֶת שֶׁהוּא נוֹתֵן לָהּ, וְאֶת שֶׁהוּא נוֹטֵל מִמֶּנָּה, מִפְּנֵי שֶׁחֲשׁוּדָה לַחֲלֹף. אָמַר רַבִּי יוֹסֵי, אֵין אָנוּ אַחֲרָאִין לָרַמָּאִין, אֵינוֹ מְעַשֵּׂר אֶלָּא מַה שֶּׁהוּא נוֹטֵל מִמֶּנָּה בִּלְבָד:

One who gives [his tithed produce] to an innkeeper [so that she can prepare it for him] must tithe from what he gives to her and from what he receives [back] from her because she is suspected of exchanging [her <i>Demai</i> for his tithed produce]. Said Rabbi Yossi, "We are not responsible for deceivers; he does not tithe except for what he receives from her exclusively."

Jerusalem Talmud Maasrot