Bhagavad Gita 17.26

Sat' means Reality or the highest Good, and also, O Arjuna, it is used to denote an action of exceptional merit.

सद्भावे साधुभावे च सदित्येतत्प्रयुज्यते।प्रशस्ते कर्मणि तथा सच्छब्दः पार्थ युज्यते॥

sadbhāve sādhubhāve ca sadityetatprayujyate|praśaste karmaṇi tathā sacchabdaḥ pārtha yujyate||

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