Reference for Demai 2:2
הַמְקַבֵּל עָלָיו לִהְיוֹת נֶאֱמָן, מְעַשֵּׂר אֶת שֶׁהוּא אוֹכֵל, וְאֶת שֶׁהוּא מוֹכֵר, וְאֶת שֶׁהוּא לוֹקֵחַ, וְאֵינוֹ מִתְאָרֵחַ אֵצֶל עַם הָאָרֶץ. רַבִּי יְהוּדָה אוֹמֵר, אַף הַמִּתְאָרֵחַ אֵצֶל עַם הָאָרֶץ נֶאֱמָן. אָמְרוּ לוֹ, עַל עַצְמוֹ אֵינוֹ נֶאֱמָן, כֵּיצַד יְהֵא נֶאֱמָן עַל שֶׁל אֲחֵרִים:
One who takes it on himself to be trustworthy must tithe that which he eats, and that which he sells, and that which he buys, and he may not stay as a guest of an <i>Am HaAretz</i> [one who is lax in observing tithes and purity laws]. Rabbi Yehudah says, “Even if he is a guest of an <i>Am HaAretz</i>, he is trustworthy.” They said to him, “[If] for his own consumption he is not trustworthy, how can he be trustworthy with that which belongs to another?”