SB 7.11.15

vaiśyas tu vārtā-vṛttiḥ syān
 nityaṁ brahma-kulānugaḥ
śūdrasya dvija-śuśrūṣā
 vṛttiś ca svāmino bhavet
Synonyms: 
vaiśyaḥ — the mercantile community; tu — indeed; vārtā-vṛttiḥ — engaged in agriculture, cow protection, and trade; syāt — must be; nityam — always; brahma-kula-anugaḥ — following the directions of the brāhmaṇas; śūdrasya — of the fourth-grade persons, the workers; dvija-śuśrūṣā — the service of the three higher sections (the brāhmaṇas, kṣatriyas and vaiśyas); vṛttiḥ — means of livelihood; ca — and; svāminaḥ — of the master; bhavet — he must be.
Translation: 
The mercantile community should always follow the directions of the brāhmaṇas and engage in such occupational duties as agriculture, trade, and protection of cows. For the śūdras the only duty is to accept a master from a higher social order and engage in his service.