Bhagavad Gita 17.27

Conviction in sacrifice, austerity, and giving is also called Sat. So too, an action done only for the Lord's sake.

यज्ञे तपसि दाने च स्थितिः सदिति चोच्यते।कर्म चैव तदर्थीयं सदित्येवाभिधीयते॥

yajñe tapasi dāne ca sthitiḥ saditi cocyate|karma caiva tadarthīyaṃ sadityevābhidhīyate||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).