One verse

Srimad Bhagavatam 11.17.48

11.17.48

वैश्यवृत्त्या तु राजन्यो जीवेन्मृगययापदि । चरेद् वा विप्ररूपेण न श्ववृत्त्या कथञ्चन ॥ ४८ ॥

vaiśya-vṛttyā tu rājanyo jīven mṛgayayāpadi cared vā vipra-rūpeṇa na śva-vṛttyā kathañcana

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
vā
or
vipra-rūpeṇa
in the form of a brāhmaṇa
na
never
śva
of the dog
vṛttyā
by the profession
kathañcana
in any circumstance

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.