Śraddhā-Trayā-Vibhāga Yoga · Verse 22
ॐतत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः | ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा
oṃ tat sad iti nirdeśo brahmaṇas tri-vidhaḥ smṛtaḥ | brāhmaṇā tena vedāś ca yajñāś ca vihitāḥ purā
‘Oṃ, Tat, Sat’ — this triple designation of Brahman is remembered. By it were ordained in the past the brāhmaṇas, the Vedas and the sacrifices.
The three symbols of Brahman: oṃ (primordial syllable), tat (That, the transcendent Brahman), sat (Being, Reality). They are nirdeśa — designation, indication of the indesignable.
By them were vihitāḥ — ordained, instituted — in the past (purā): brāhmaṇāḥ (priests/scholars), vedāḥ (scriptures), yajñāḥ (sacrifices). All dharma is established in reference to these three.