Principal Upanishad · Samaveda
Chandogya Upanishad 6.8.7
Tat tvam asi — thou art that
स य एषोऽणिमैतदात्म्यमिदं सर्वं तत्सत्यं स आत्मा तत्त्वमसि।
That which is the subtle essence — in That all that exists has its self. That is the Truth. That is the Self. Thou art That.
What this verse points to
One of the four mahavakyas, the "great statements," and the culmination of Uddalaka's instruction to his son. It is repeated nine times in the chapter, each time after a different illustration — salt dissolved in water, rivers entering the sea, sap running through a tree.
The repetition tells you how the text expected to be used. It is not an argument to be accepted once but a statement to be returned to until it stops being information.
Sit with it
Sit with the sentence and, rather than agreeing with it, look for what in you resists it.
About the Chandogya Upanishad
One of the oldest and longest Upanishads. Its sixth chapter contains the instruction of Uddalaka Aruni to his son Shvetaketu, culminating in the mahavakya tat tvam asi — "thou art that" — repeated nine times with a different illustration each time.
References to this text are given as chapter.section.verse — here, 6.8.7.