Mahabharata Bhishma Parva Chapter 41:2

Mahabharata Bhishma Parva (Bhagavat-Gita Parva) Chapter 41:2 Bhagavad Gita Chapter XVII The speech which causeth no agitation, which is true, which is agreeable and beneficial, and the diligent study of the Vedas, are said to be the penance of speech. Serenity of the mind, gentleness, taciturnity, self-restraint, and purity of the disposition,—these are said to be the penance of the mind. This three-fold penance performed with perfect faith, by men without desire of fruit, and with devotion, is said to be of the quality of goodness. That penance which is performed for the sake of (gaining) respect, honour, and reverence, with hypocrisy, (and) which is unstable and transient is said to be of the quality of passion. That penance which is performed under a deluded conviction, with torture of one's self, and for the destruction of another, is said to be of the quality of darkness. That gift which is given because it ought to be given, to one who cannot return any service for it, in a proper time, and to a proper person, is said to be of the quality of goodness. That, however, which is given reluctantly, for return of services (past or expected), or even with an eye to fruit,—that gift is said to be of the quality of passion. In an unfit place and at an unfit time, the gift that is made to an unworthy object, without respect, and with contempt, is said to be of the quality of darkness. OM, TAT, SAT, this is said to be the three-fold designation of Brahma. By that (Brahma), the Brahmanas and the Vedas, and the Sacrifices, were ordained of old. Therefore, uttering the syllable OM, the sacrifices, gifts, and penances, prescribed by the ordinance, of all utterers of Brahma begin. Uttering TAT, the various rites of sacrifice, penance, and gifts, without expectation of fruit, are performed by those that are desirous of deliverance. SAT is employed to denote existence and goodness. Likewise, O son of Pritha, the word SAT is used in any auspicious act. Constancy in sacrifices, in penances and in gifts, is also called SAT, and an act, too, for the sake of That is called SAT. Whatever oblation is offered (to the fire), whatever is given away, whatever penance is performed, whatever is done, without faith, is, O son of Pritha, said to be the opposite of SAT; and that is nought both here and hereafter.