Adhyāya 17 • Shloka 23

ॐ तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः | ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा || २३ ||

oṁ tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥ | brāhmaṇās tena vedāś ca yajñāś ca vihitāḥ purā || 23 ||

"From the beginning of creation, the three words om tat sat were used to indicate the Supreme Absolute Truth. These three symbolic representations were used by brāhmaṇas while chanting the hymns of the Vedas and during sacrifices for the satisfaction of the Supreme."