SB 7.11.15
vaisyas tu varta-vrttih syan
nityam brahma-kulanugah
sudrasya dvija-susrusa
vrttis ca svamino bhavet
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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.