Chandogya Upanishad 3.12

The Gâyatrî (verse) is everything whatsoever here exists. Gâyatrî indeed is speech, for speech sings forth (gâya-ti) and protects (trâya-te) everything that here exists.

That Gâyatrî is also the earth, for everything that here exists rests on the earth, and does not go beyond.

That earth again is the body in man, for in it the vital airs (prânas, which are everything) rest, and do not go beyond.

That body again in man is the heart within man, for in it the prânas (which are everything) rest, and do not go beyond.

That Gâyatrî has four feet and is sixfold. And this is also declared by a Rik verse (Rig-veda X, 90, 3):—

'Such is the greatness of it (of Brahman, under the disguise of Gâyatrî); greater than it is the Person (purusha). His feet are all things. The immortal with three feet is in heaven (i.e. in himself).'

The Brahman which has been thus described (as immortal with three feet in heaven, and as Gâyatrî) is the same as the ether which is around us;

And the ether which is around us, is the same as the ether which is within us. And the ether which is within us,

That is the ether within the heart. That ether in the heart (as Brahman) is omnipresent and unchanging. He who knows this obtains omnipresent and unchangeable happiness.