Vishnupurana 3.8.40.p1

3.8.40.p1

iti śrīviṣṇupurāṇe tṛtīyāṃśe 'ṣṭamo 'dhyāyaḥ

"In times of distress the peculiar functions of the castes may be modified, as you shall hear. A Brahman may follow the occupations of a Kshatriya or a Vaiśya; the Kshatriya may adopt those of the Vaiśya; and the Vaiśya those of the Kshatriya: but these two last should never descend to the functions of the Śúdra, if it be possible to avoid them; and if that be not possible, they must at least shun the functions of the mined castes. I will now, Rájá, relate to you the duties of the several Ásramas or conditions of life."