B.G 17.23

ॐ तत् सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः। ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा ॥२३॥

oṃ tat saditi nirdeśo brahmaṇastrividhaḥ smṛtaḥ। brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā ॥23॥

[ॐ (oṃ) - Om; तत् (tat) - that; सत् (sat) - truth; इति (iti) - thus; निर्देशः (nirdeśaḥ) - indication; ब्रह्मणः (brahmaṇaḥ) - of Brahman; त्रिविधः (trividhaḥ) - threefold; स्मृतः (smṛtaḥ) - is remembered; ब्राह्मणाः (brāhmaṇāḥ) - the Brahmanas; तेन (tena) - by that; वेदाः (vedāḥ) - the Vedas; च (ca) - and; यज्ञाः (yajñāḥ) - sacrifices; च (ca) - and; विहिताः (vihitāḥ) - were ordained; पुरा (purā) - in ancient times;]

(Om, that, truth - thus is the threefold indication of Brahman remembered. By that, the Brahmanas, the Vedas, and sacrifices were ordained in ancient times.)

By the directive "Om, Tat, Sat", the three aspects of Brahman is remembered. By this, the Brahmanas, the Vedas, and sacrifices were instituted in ancient times.