BG 18.43-45
Prowess, splendour, firmness, dexterity, and also not flying from battle, generosity, the nature of a ruler, are the Kshattriya duty, born of his own nature. Ploughing, protection of kine, and trade are the Vaishya duty, born of his own nature. Action of the nature of service is the Shûdra duty, born of his own nature. Man reacheth perfection by each being intent on his own duty. Listen thou how perfection is won by him who is intent on his own duty.