Bhagavad Gita · 18.45

स्वे स्वे कर्मण्यभिरतः संसिद्धिं लभते नरः।स्वकर्मनिरतः सिद्धिं यथा विन्दति तच्छृणु॥

sve sve karmaṇy abhirataḥ saṁsiddhiṁ labhate naraḥ sva-karma-nirataḥ siddhiṁ yathā vindati tach chhṛiṇu

By being devoted to one's own duty, a person attains perfection. Hear how one devoted to one's own duty attains perfection.

— Bhagavad Gita 18.45

Source: Sacred-Texts.com (Sanskrit) + Bhagavad-Gita.io (translations)