Reference for Peah 8:4

נֶאֱמָנִים עַל הַיָּרָק חַי, וְאֵין נֶאֱמָנִים עַל הַמְבֻשָּׁל, אֶלָּא אִם כֵּן הָיָה לוֹ דָּבָר מֻעָט, שֶׁכֵּן דֶּרֶךְ בַּעַל הַבַּיִת לִהְיוֹת מוֹצִיא מִלְּפָסוֹ:

[Poor] people are trusted regarding raw vegetables but not trusted regarding cooked vegetables, unless he had a small amount, because it is common that the property owner takes it out from his stew pot.

Jastrow