Bhagavad Gita · 18.41

ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप।कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः॥

brāhmaṇa-kṣhatriya-viśhāṁ śhūdrāṇāṁ cha parantapa karmāṇi pravibhaktāni svabhāva-prabhavair guṇaiḥ

O scorcher of foes, the duties of Brahmins, Kshatriyas, Vaishyas, and Shudras are distributed according to the qualities born of their own nature.

— Bhagavad Gita 18.41

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)