Bhagavad Gita 17.23

ॐ तत्सदिति निर्देशो ब्राह्मणस्त्रिविध: स्मृत: |
ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिता: पुरा

Translation

The words “Om Tat Sat” have been declared as symbolic representations of the Supreme Absolute Truth, from the beginning of creation. From them came the priests, scriptures, and sacrifice.