अफलाकाङ्क्षिभिर्यज्ञो विधिदृष्टो य इज्यते। यष्टव्यमेवेति मनः समाधाय स सात्त्विकः ॥

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

The sacrifice which is offered by men without desire for fruit, as enjoined by the ordinances, under the firm belief that sacrifice is a duty, that is pure.

In plain words: Worship or offering is pure when people do it the right way simply because it is their duty, without wanting anything back.