Bhagavad Gita 17.28

यज्ञे तपसि दाने च स्थितः सदिति चोच्यते |
कर्म चैव तदर्थेयं सदितयेवाभिधीयते

Translation

Being established in the performance of sacrifice, penance, and charity, is also described by the word “Sat.” And so any act for such purposes is named “Sat.”