Bhagavad-gītā As It Is · Text 18.41
Natural duties and perfection through work
ब्राह्मणक्षत्रियविशां शूद्राणां च परन्तप ।
कर्माणि प्रविभक्तानि स्वभावप्रभवैर्गुणै: ॥ ४१ ॥
brāhmaṇa-kṣatriya-viśāṁ
śūdrāṇāṁ ca paran-tapa
karmāṇi pravibhaktāni
svabhāva-prabhavair guṇaiḥ
Brāhmaṇas, kṣatriyas, vaiśyas and śūdras are distinguished by the qualities born of their own natures in accordance with the material modes, O chastiser of the enemy.
Read Śrīla Prabhupāda's purport on Vedabase.io