77٩
Bhagavad-ojja-As  Jt Is
[Ch 17]
om-tat-sad iti nirdeśo  
 brahmaṇas tri-vidhaḥ smṛtaḥ  
 brāhmaṇās tena vedāś ca  
 vaiñāś ca vihitāh purā
om —indication of the Supreme;  tat —that;  sat —eternal;  iti —that;  nirdeśaḥ —indication;  brāhmaṇāḥ —of the Supreme;  tri-vidhaḥ —three kinds;  smṛtaḥ —consider;  brahmaṇaḥ —the  brāhmaṇas ;  tena —therefore;  vedāḥ —the Vedic literature;  ca —also;  yajñāḥ —sacrifice;  ca —also;  vihitāḥ —sacrifice;  purā —formerly.
TRANSLATION
From the beginning of creation, the three syllables—om tat sat—have been used to indicate the Supreme Absolute Truth [Brahman]. They were uttered by brāhmaṇas while chanting Vedic hymns and during sacrifices, for the satisfaction of the Supreme.
PURPORT
It has been explained that penance, sacrifice, charity and foods are divided into three categories: the modes of goodness, passion and ignorance. But whether first class, second class or third class, they are all conditioned, contaminated by the material modes of nature. When they are aimed at the Supreme— om tat sat , the Supreme Personality of Godhead, the eternal—they become means for spiritual elevation. In the scriptural injunctions such an objective is indicated. These three words,  om tat sat , particularly indicate the Absolute Truth, the Supreme Personality of Godhead. In the Vedic hymns, the word  om  is always found.
One who acts without following the regulations of the scriptures will not attain the Absolute Truth. He will get some temporary result, but not the ultimate end of life. The conclusion is that the performance of charities, sacrifice and penance must be done in the mode of goodness. Performed in the modes of passion or ignorance, they are certainly inferior in quality. The three words  om tat sat  are uttered in conjunction with the holy name of the Supreme Lord, e.g.,  om tad viṣṇoḥ . Whenever a Vedic hymn or the holy name of the Supreme Lord is uttered,  om  is added. This is the indication of Vedic literature. These three words are taken from Vedic hymns.  Om ity etad brahmaṇo nediṣṭaṁ nāma  indicates