Bhagavad Gita · 17.27

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

yajñe tapasi dāne cha sthitiḥ sad iti chochyate karma chaiva tad-arthīyaṁ sad ity evābhidhīyate

Steadfastness in sacrifice, austerity, and charity is also called 'Sat.' And action performed for the sake of the Supreme is likewise designated as 'Sat.'

— Bhagavad Gita 17.27

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