Bava Metzia6: 6
כָּל הָאֻמָּנִין שׁוֹמְרֵי שָׂכָר הֵן, וְכֻלָּן שֶׁאָמְרוּ: ,,טֹל אֶת שֶׁלְּךָ וְהָבֵא מָעוֹת” — שׁוֹמֵר חִנָּם. ,,שְׁמֹר לִי וְאֶשְׁמֹר לָךְ” — שׁוֹמֵר שָׂכָר. ,,שְׁמֹר לִי”, וְאָמַר לוֹ: ,,הַנַּח לְפָנָי” — שׁוֹמֵר חִנָּם.
All craftsmen are regarded as paid shomrim, and any of them who says: “Take your [article] and bring money” is an unpaid shomer. [If one says:] “Watch for me and I will watch for you,” he is a paid shomer. [If one says:] “Watch for me,” and he re-plies: “Put it down before me,” he is an unpaid shomer.