Sraddhatraya Vibhaga Yoga-Bhagavad Gita Chapter 17 [The Three Divisions of Material Existence]
Krishna Said > Shaloka: 26
The Absolute Truth is the objective of devotional sacrifice, and it is indicated by the word sat. These works of sacrifice, of penance and of charity, true to the absolute nature, are performed to please the Supreme Person, O son of Prtha.
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 parental care 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 should vibrate om tat sat. The words sad-bhave and sadhu-bhave indicate the transcendental situation. One who is acting in Krishna consciousness is called sattva, and one who is fully conscious of activities in Krishna consciousness is called svarupa. In the Srimad-Bhagavatam it is said that the transcendental subject matter becomes clear in the association of the devotees. 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 yajna performances, the supreme object, om tat sat is invoked. These words om tat sat are used to perfect all activities. The supreme om tat sat makes everything complete.