Bhagavad Gita Chapter 17.23
oḿ tat sad iti nirdeśoTranslation
brahmaṇas tri-vidhaḥ smṛtaḥ
brāhmaṇās tena vedāś ca
yajñāś ca vihitāḥ purā
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 brahmanas while chanting the hymns of the Vedas and during sacrifices for the satisfaction of the Supreme.