One verse

Srimad Bhagavatam 7.11.15

7.11.15

वैश्यस्तु वार्तावृत्ति: स्यान्नित्यं ब्रह्मकुलानुग: । शूद्रस्य द्विजशुश्रूषा वृत्तिश्च स्वामिनो भवेत् ॥ १५ ॥

vaisyas tu varta-vrttih syan nityam brahma-kulanugah sudrasya dvija-susrusa vrttis ca svamino bhavet

vaisyah
the mercantile community
tu
indeed
varta-vrttih
engaged in agriculture, cow protection, and trade
syat
must be
nityam
always
brahma-kula-anugah
following the directions of the brahmanas
sudrasya
of the fourth-grade persons, the workers
dvija-susrusa
the service of the three higher sections (the brahmanas, ksatriyas and vaisyas)
vrttih
means of livelihood
ca
and
svaminah
of the master
bhavet
he must be

The mercantile community should always follow the directions of the brahmanas and engage in such occupational duties as agriculture, trade, and protection of cows. For the sudras the only duty is to accept a master from a higher social order and engage in his service.