Mahabharata 3.149.35

3.149.35

yājanādhyāpane cobhe brāhmaṇānāṃ pratigrahaḥ |
pālanaṃ kṣatriyāṇāṃ vai vaiśyadharmaśca poṣaṇam || 35 ||

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. Then only is the order of the world secured, when the king duly punisheth and conferreth favours.