Bhagavad Gita 17.11
Sacrifice is pure when it is offered by one who does not covet the fruit of it, when it is done according to the commands of scripture, and with implicit faith that the sacrifice is a duty.
अफलाकाङ्क्षिभिर्यज्ञो विधिदृष्टो य इज्यते।यष्टव्यमेवेति मनः समाधाय स सात्त्विकः॥aphalākāṅkṣibhiryajño vidhidṛṣṭo ya ijyate|yaṣṭavyameveti manaḥ samādhāya sa sāttvikaḥ||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).