Mbh 3.149.4b

The gods are maintained by the (celebration of) sacrifices prescribed by the Vedas and the religious ordinances; while men maintain themselves by (following) the ordinances of Vrihaspati and Usanas and also by these avocations, by which the world is maintained,--serving for wages, (receiving) taxes, merchandise, agriculture and tending kine and sheep. The world subsisteth by profession. The (study of the) three Vedas and agriculture and trade and government constitutes, it is ordained by the wise, the professions of the twice born ones; and each order maintaineth itself by following the profession prescribed for it. And when these callings are properly pursued, the world is maintained with ease. If, however, people do not righteously lead their lives, the world becometh lawless, in consequence of the want of Vedic merit and government. And if people do not resort to (their) prescribed vocations, they perish, but by regularly following the three professions, they bring about religion. The religion of the Brahmanas consisteth in the knowledge of the soul and the hue of that order alone is universally the same. The celebration of sacrifices, and study and bestowal of gifts are well-known to be the three duties common (to all these orders). Officiating at sacrifices, teaching and the acceptance of gifts are the duties of a Brahmana. To rule (the subjects) is the duty of the Kshatriya; and to tend (cattle), that of the Vaisya, while to serve the twice-born orders is said to be the duty of the Sudra. The Sudras cannot beg alms, or perform homas, or observe vows; and they must dwell in the habitation of their masters. Thy vocation, O son of Kunti, is that of the Kshatriya, which is to protect (the subjects). Do thou carry out thy own duties, in an humble spirit, restraining thy senses. That king alone can govern, who taketh counsel of experienced men, and is helped by honest, intelligent and learned ministers; but a king who is addicted to vices, meeteth with defeat.