Tat tvam asi, “that you are”, is repeated nine times in the sixth chapter of the Chāndogya Upaniṣad, where the sage Uddālaka teaches his son Śvetaketu with images of salt dissolved in water and of the seed of the banyan. “That” is the subtle essence of all things; “you” is the self. The schools of Vedānta are distinguished by how they read the identity.
Tat tvam asi
“That thou art”: the great saying of the Chāndogya Upaniṣad that the self is one with the ground of all.
Also: That thou art