Chandogya Upanishad

The finest essence here— :that constitutes the self of this whole world; :that is the truth; :that is the self. :Tat tvam asi (that's how you are).

The Chandogya Upanishad is one of the principal Upanishads.

Quotes

Olivelle, Patrick (1998). The Early Upanishads. Oxford: Oxford University Press. ISBN 0-19-512435-9. 

• This self (atman) of mine that lies deep within my heart—

it is smaller than a grain of rice or barley,

smaller than a mustard seed,

smaller even than a millet grain or a millet kernel;

but it is larger than the earth,

larger than the intermediate region,

larger than the sky,

larger even than all these worlds put together.

This self (atman) of mine that lies deep within my heart—

it contains all actions, all desires, all smells, and all tastes;

it has captured this whole world;

it neither speaks nor pays any heed.

It is brahman.

On departing from here after death, I will become that.

• 3.14

• It is like this. Take a bird that is tied with a string. It will fly off in every direction and, when it cannot find a resting place anywhere else, it will alight back upon the very thing to which it is tied.

Similarly, son, the mind flies off in every direction and, when it cannot find a resting place anywhere else, it alights back upon the breath itself; for the mind, my son, is tied to the breath.

• 6.8.2

• The finest essence here—

that constitutes the self of this whole world;

that is the truth;

that is the self (atman).

And that's how you are (tat tvam asi), Śvetaketu.

• 6.8.8–15

• So, those here in this world who depart without having discovered the self and these real desires

do not obtain complete freedom of movement in any of the worlds,

whereas those here in this world who depart after discovering the self and these real desires

obtain complete freedom of movement in all the worlds.

• 8.1.6