Chapter 17 · The Three Kinds of Faith · Krishna said
Bhagavad Gita 17.23 — Om Tat Sat
ॐतत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः। ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥
OMtatsaditi nirdeśo brahmaṇastrividhaḥ smṛtaḥ | brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā ||
Meaning
'Om Tat Sat' is said to be the threefold name of Brahman. Through it, in ancient times, the brahmins, the Vedas and sacrifices were created.