Bhagwad Gita
Chapter 18, Verse 41
ब्राह्मणक्षत्रियविशां शूद्राणां च परंतप।कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणैः।।18.41।।
Brahmanas, ksatriyas, vaisyas and sudras are distinguished by their qualities of work, O chastiser of the enemy, in accordance with the modes of nature.