Shiva Purana

siva.sh/siva-purana/uma-samhita/21/8

कर्मणा येन मेधावी शूद्रो वैश्यो हि जायते ।। तत्ते वक्ष्यामि निखिलं येन वर्णोत्तमो भवेत् ।। ८ ।।

sanskrit

I shall explain to you entirely that which gives a higher caste whereby an intelligent Śūdra becomes a Vaiśya.

english translation

karmaNA yena medhAvI zUdro vaizyo hi jAyate || tatte vakSyAmi nikhilaM yena varNottamo bhavet || 8 ||

hk transliteration by Sanscript