Reference for Demai 6:12

בעַם הָאָרֶץ שֶׁאָמַר לְחָבֵר, קַח לִי אֲגֻדַּת יָרָק, קַח לִי גְלֻסְקָן אֶחָד, לוֹקֵחַ סְתָם וּפָטוּר. וְאִם אָמַר, שֶׁלִּי זֶה, וְזֶה שֶׁל חֲבֵרִי, וְנִתְעָרְבוּ, חַיָּב לְעַשֵּׂר, וַאֲפִלּוּ הֵן מֵאָה:

An <i>Am HaAretz</i> who says to a <i>Chaver</i>, "Buy me a bundle of vegetables", "buy me one loaf": he buys without specifying and is exempt from tithing. If he [buys for himself as well and] says "this is mine and this is my friend's" and they become mixed, he is obligated to tithe, even if [he bought] 100 items.

Jastrow