Bhagavad Gita 18.41

ब्राह्मणक्षत्रियविषां शूद्राणां च परन्तप |
कर्माणि प्रविभक्तानि स्वभावप्रभावैर्गुणै:

Translation

The duties of the Brahmins, Kshatriyas, Vaishyas, and Shudras—are distributed according to their qualities, in accordance with their guṇas (and not by birth).