Bhagavad Gita 17.23

ॐ तत्सदिति निर्देशो ब्रह्मणस्त्रिविध: स्मृत: ।
ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिता: पुरा ॥

OM tat-sad-iti nirdeśo brahmaṇas-trividha: smṛta:
brāhmaṇās-tena vedāś-ca yajñāś-ca vihitā: purā – 23

OM, TAT and SAT are spoken of as the triple attributes of Brahman, the supreme Reality. By these were ordained the Brāhmaṇas, Vedas and yajñas, in the ancient times.