Bhagavad Gita 17.27
यज्ञे तपसि दाने च स्थितिः सदिति चोच्यते। कर्म चैव तदर्थीयं सदित्येवाभिधीयते।।17.27।।
yajñe tapasi dāne ca sthitiḥ saditi cocyate| karma caiva tadarthīyaṃ sadityevābhidhīyate||17.27||
yajne tapasi dane ca sthitih saditi cocyate karma caiva tadarthiyam sadityevabhidhiyate17.27
Meaning
Steadfastness in sacrifice, austerity, and charity is also called 'Sat.' Any action performed for that purpose — of sacrifice, austerity, and charity — is also called 'Sat.'
यज्ञ, तप और दान में दृढ़ स्थिति भी सत् कही जाती है, और उस के लिए किया गया कर्म भी सत् ही कहलाता है।
This is verse 27 of 28 in Chapter 17 (Shraddhatraya Vibhaga Yoga) of the Bhagavad Gita.