SB 11.17.48

vaiśya-vṛttyā tu rājanyo
 jīven mṛgayayāpadi
cared vā vipra-rūpeṇa
 na śva-vṛttyā kathañcana
Synonyms: 
vaiśya — of the mercantile class; vṛttyā — by the occupation; tu — indeed; rājanyaḥ — a king; jīvet — may maintain himself; mṛgayayā — by hunting; āpadi — in an emergency or disastrous situation; caret — may act; — or; vipra-rūpeṇa — in the form of a brāhmaṇa; na — never; śva — of the dog; vṛttyā — by the profession; kathañcana — in any circumstance.
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.