Srimad-Bhagavatam: Canto 10 - Chapter 1 - Verse 20

Sanskrit:

वर्तेत ब्रह्मणा विप्रो राजन्यो रक्षया भुव: । वैश्यस्तु वार्तया जीवेच्छूद्रस्तु द्विजसेवया ॥ २० ॥

ITRANS:

varteta brahmaṇā viprorājanyo rakṣayā bhuvaḥvaiśyas tu vārtayā jīvecchūdras tu dvija-sevayā

Translation:

The brāhmaṇa maintains his life by studying and teaching the Vedas, the member of the royal order by protecting the earth, the vaiśya by trade, and the śūdra by serving the higher, twice-born classes.

Purport:

After glorifying karma, or work, Lord Kṛṣṇa 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 varṇāśrama, or Vedic social system.