Srimad-Bhagavatam: Canto 11 - Chapter 1 - Verse 48
Sanskrit:
वैश्यवृत्त्या तु राजन्यो जीवेन्मृगययापदि ।चरेद् वा विप्ररूपेण न श्ववृत्त्या कथञ्चन ॥ ४८ ॥
ITRANS:
vaiśya-vṛttyā tu rājanyojīven mṛgayayāpadicared vā vipra-rūpeṇana śva-vṛttyā kathañcana
Translation:
A king or other member of the royal order who cannot maintain himself by his normal occupation may act as a vaiśya, may live by hunting or may act as a brāhmaṇa by teaching others Vedic knowledge. But he may not under any circumstances adopt the profession of a śūdra.