Bg 17.26-27

sad-bhave sadhu-bhave ca
sad ity etat prayujyate
prasaste karmani tatha
sac-chabdah partha yujyate

yajne tapasi dane ca
sthitih sad iti cocyate
karma caiva tad-arthiyam
sad ity evabhidhiyate

Word for word: 
sat-bhave — in the sense of the nature of the Supreme; sadhu-bhave — in the sense of the nature of the devotee; ca — also; sat — the word sat; iti — thus; etat — this; prayujyate — is used; prasaste — in bona fide; karmani — activities; tatha — also; sat-sabdah — the sound sat; partha — O son of Prtha; yujyate — is used; yajne — in sacrifice; tapasi — in penance; dane — in charity; ca — also; sthitih — the situation; sat — the Supreme; iti — thus; ca — and; ucyate — is pronounced; karma — work; ca — also; eva — certainly; tat — for that; arthiyam — meant; sat — the Supreme; iti — thus; eva — certainly; abhidhiyate — is indicated.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 Prtha.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The words prasaste karmani, or “prescribed duties,” indicate that there are many activities prescribed in the Vedic literature which are purificatory processes, beginning from the time of conception up to the end of one’s life. Such purificatory processes are adopted for the ultimate liberation of the living entity. In all such activities it is recommended that one vibrate om tat sat. The words sad-bhave and sadhu-bhave indicate the transcendental situation. Acting in Krishna consciousness is called sattva, and one who is fully conscious of the activities of Krishna consciousness is called a sadhu. In the Srimad-Bhagavatam (3.25.25) it is said that the transcendental subject matter becomes clear in the association of the devotees. The words used are satam prasangat. Without good association, one cannot achieve transcendental knowledge. When initiating a person or offering the sacred thread, one vibrates the words om tat sat. Similarly, in all kinds of performance of yajna the object is the Supreme, om tat sat. The word tad-arthiyam further means offering service to anything which represents the Supreme, including such service as cooking and helping in the Lord’s temple, or any other kind of work for broadcasting the glories of the Lord. These supreme words om tat sat are thus used in many ways to perfect all activities and make everything complete.