Bhagavad Gita 17.23

Om Tat Sat is the triple designation of the Eternal Spirit, by which the Vedic Scriptures, ceremonials, and sacrifices were ordained of old.

तत्सदिति निर्देशो ब्रह्मणस्त्रिविधः स्मृतः।ब्राह्मणास्तेन वेदाश्च यज्ञाश्च विहिताः पुरा॥

tatsaditi nirdeśo brahmaṇastrividhaḥ smṛtaḥ|brāhmaṇāstena vedāśca yajñāśca vihitāḥ purā||

English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).