The Law Code of Yājñavalkya 3.2.5

3.2.5

tathā āpady api na hīnavarṇena brāhmī vṛttir āśrayaṇīyā kiṃ tu brāhmaṇena kṣātrī kṣatriyeṇa vaiśyasaṃbandhinī vaiśyena ca śaudrīty evaṃ svānantarahīnavarṇavṛttir eva | ajīvantaḥ svadharmeṇāntarāṃ pāpīyasīṃ vṛttim ātiṣṭherann atu kadācij jyāyasīm (VaDh 2.22–23) iti vasiṣṭhasmaraṇāt | jyāyasī ca brāhmī vṛttiḥ | tathā ca smṛtyantaram |

Likewise, even in distress, the livelihood of a Brahmin is not to be resorted to by a lower class; rather, a Brahmin [may resort] to that of a Kshatriya, a Kshatriya to that belonging to a Vaishya, and a Vaishya to that of a Shudra—thus only the livelihood of the lower class immediately next to one's own. For Vasiṣṭha records: "Not surviving by their own duty, they may take up the livelihood that is immediately lower, but never a higher one" (VaDh 2.22–23). And the livelihood of a Brahmin is higher. And likewise another legal text: