Bhagavad Gita · 17.11

अफलाकाङ्क्षिभिर्यज्ञो विधिदिष्टो य इज्यते।यष्टव्यमेवेति मनः समाधाय स सात्त्विकः॥

aphalākāṅkṣhibhir yajño vidhi-diṣhṭo ya ijyate yaṣhṭavyam eveti manaḥ samādhāya sa sāttvikaḥ

Sacrifice that is performed according to scriptural guidelines, by those who expect no reward, with the firm conviction that it is one's duty — that sacrifice is in the mode of goodness.

— Bhagavad Gita 17.11

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