Bhagavad Gita 18.41
O Arjuna! The duties of spiritual teachers, the soldiers, the traders, and the servants have all been fixed according to the dominant quality in their nature.
ब्राह्मणक्षत्रियविशां शूद्राणां च परंतप।कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः॥brāhmaṇakṣatriyaviśāṃ śūdrāṇāṃ ca paraṃtapa|karmāṇi pravibhaktāni svabhāvaprabhavairguṇaiḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).