oṁ tat sad iti nirdeśho brahmaṇas tri-vidhaḥ smṛitaḥ
brāhmaṇās tena vedāśh cha yajñāśh cha vihitāḥ purā
om tat sat - syllables representing aspects of transcendence
iti - thus
nirdeśhaḥ - symbolic representatives
brahmaṇaḥ - the Supreme Absolute Truth
tri-vidhaḥ - of three kinds
smṛitaḥ - have been declared
brāhmaṇāḥ - the priests
tena - from them
vedāḥ - scriptures
cha - and
yajñāḥ - sacrifice
cha - and
vihitāḥ - came about
purā - from the beginning of creation
Translation
"Om Tat Sat": This has been declared to be the triple designation of Brahman. By that, the Brahmanas, the Vedas, and the sacrifices were created formerly.