B.G 17.27 and 28
yajñe tapasi dāne ca sthitiḥ saditi cocyate। karma caiva tadarthīyaṃ sadityevābhidhīyate ॥27॥
Steadiness in sacrifice, austerity, and charity is referred to as 'sat', i.e. real, and so also is the action taken for that purpose.
aśraddhayā hutaṃ dattaṃ tapastaptaṃ kṛtaṃ ca yat। asadityucyate pārtha na ca tat pretya no iha ॥28॥
O son of Pritha, whatever is offered, given, or performed without faith is considered 'asat', i.e. unreal. It benefits us neither in this life nor in the afterlife.
॥ oṁ tatsaditi śrīmadbhagavadgītāsu upaniṣatsu brahmavidyāyāṁ yogaśāstre śrīkṛṣṇārjunasaṁvāde śraddhātrayavibhāgayogo nāma saptadaśādhyāyaḥ ॥
'Om Tat Sat'. Thus, the seventeenth chapter of the Bhagavad Gita, named 'The Yoga of the Division of the Threefold Faith', is declared in the Upanishads, in the knowledge of Brahman, and in the science of Yoga, through the dialogue between Sri Krishna and Arjuna ends.