SB 10.24.20
varteta brahmana vipro
rajanyo raksaya bhuvah
vaisyas tu vartaya jivec
chudras tu dvija-sevaya
Translation:
The brahmana maintains his life by studying and teaching the Vedas, the member of the royal order by protecting the earth, the vaisya by trade, and the sudra by serving the higher, twice-born classes.
Purport:
After glorifying karma, or work, Lord Krsna now explains what He means by prescribed duties born of one’s nature. He was not referring to any whimsical activity, but rather to the religious duties prescribed in the varnasrama, or Vedic social system.