BG 17.26
The word "Sat" refers to the existence of that which is good. As such, it is used to refer to an auspicious action.
प्रशस्ते कर्मणि तथा सच्छब्दः पार्थ युज्यते
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sad-bhāve — with the intention of eternal existence and goodness
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sad
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bhāve
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sādhu-bhāve — with auspicious intention
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sādhu
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bhāve
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cha — also
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sad — the syllable Sat
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ity — thus
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etat — this
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prayujyate — is used
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praśhaste — auspicious
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karmaṇi — action
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tathā — also
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sach-chhabdaḥ — the word "Sat"
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sach
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chhabdaḥ
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pārtha — Arjuna
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yujyate — is used
"Om Tat Sat" is the threefold designation of Brahman, by which the scriptures, ceremonies, and sacrifices have been ordained. The scriptures prescribed that all acts of sacrifice, gifts, and austerities commence with the utterance of "Om". Acts of sacrifice, austerity, and charity, performed without thought of reward, are performed after uttering "Tat". The word "Sat" refers to the existence of that which is good. As such, it is used to refer to an auspicious action. Devotion to sacrifice, austerity, and charity is also referred to as "Sat", as is any action for such purposes. Acts of sacrifice, charity, or penance, when performed without faith, are called "Asat". Such acts have no significance in this world and the next.