Srimad Bhagavatam Canto 11, Chapter 17, Text 47

SB 11.17.47

sidan vipro vanig-vrttya
 panyair evapadam taret
khadgena vapadakranto
 na sva-vrttya kathañcana
 
Translation: 
 
If a brahmana cannot support himself through his regular duties and is thus suffering, he may adopt the occupation of a merchant and overcome his destitute condition by buying and selling material things. If he continues to suffer extreme poverty even as a merchant, then he may adopt the occupation of a ksatriya, taking sword in hand. But he cannot in any circumstances become like a dog, accepting an ordinary master.
 
Purport: 
 
Sva-vrttya, or “a dog’s profession,” refers to the sudras, who cannot live without accepting a master. A destitute brahmana who is suffering intolerably may become a merchant and then a ksatriya but may never take the position of a sudra by working in a company or accepting a master. Although a ksatriya is ordinarily considered more elevated than a vaisya, the Lord here recommends that distressed brahmanas first accept the vaisya occupation, since it is not violent.
Srimad Bhagavatam Canto 11, Chapter 17, Text 46
Srimad Bhagavatam Canto 11, Chapter 17, Text 48