Chapter 18 · Liberation through Renunciation · Krishna said
Bhagavad Gita 18.41 — Duty by one's nature
ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप। कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः ॥
brāhmaṇakṣatriyaviśāṃ śūdrāṇāṃ ca parantapa | karmāṇi pravibhaktāni svabhāvaprabhavairguṇaiḥ ||
Meaning
O Arjuna, the duties of brahmins, kshatriyas, vaishyas and shudras are divided according to the qualities arising from their own nature.