The Law Code of Yājñavalkya 3.2.43

3.2.43

sadyaḥ patati māṃsena lākṣayā lavaṇena ca |
tryaheṇa śūdro bhavati brāhmaṇaḥ kṣīravikrayāt ||
itareṣām apaṇyānāṃ vikrayād iha kāmataḥ |
brāhmaṇaḥ saptarātreṇa vaiśyabhāvaṃ ca gacchati ||
iti | (MDh 10.92–93) 3.40 ||

"He falls immediately by [selling] meat, lac, and salt; / a Brahmin becomes a Shudra in three days from the sale of milk. / From selling other non-vendible goods here willfully, / a Brahmin reaches the condition of a Vaishya in seven nights." (MDh 10.92–93) || 3.40