SB 11.17.48
vaisya-vrttya tu rajanyo
jiven mrgayayapadi
cared va vipra-rupena
na sva-vrttya kathañcana
Translation:
A king or other member of the royal order who cannot maintain himself by his normal occupation may act as a vaisya, may live by hunting or may act as a brahmana by teaching others Vedic knowledge. But he may not under any circumstances adopt the profession of a sudra.