Chapter 17 sloka 23 -28

TEXT 23: From the beginning of creation, the three words oṁ tat sat were used to indicate the Supreme Absolute Truth. These three symbolic representations were used by brāhmaṇas while chanting the hymns of the Vedas and during sacrifices for the satisfaction of the Supreme.
TEXT 24: Therefore, transcendentalists undertaking performances of sacrifice, charity and penance in accordance with scriptural regulations begin always with oṁ, to attain the Supreme.
TEXT 25: Without desiring fruitive results, one should perform various kinds of sacrifice, penance, and charity with the word tat. The purpose of such transcendental activities is to get free from material entanglement.
TEXTS 26-27: The Absolute Truth is the objective of devotional sacrifice, and it is indicated by the word sat. The performer of such sacrifice is also called sat, as are all works of sacrifice, penance, and charity which, true to the absolute nature, are performed to please the Supreme Person, O son of Pṛthā.
TEXT 28: Anything done as sacrifice, charity or penance without faith in the Supreme, O son of Pṛthā, is impermanent. It is called asat and is useless both in this life and the next.