Mundaka Upanishad 2:2:2

 

“He, the self-luminous, subtler than the subtlest, in whom exist all the worlds and all those that live therein–he is the imperishable Brahman. He is the principle of life. He is speech, and he is mind. He is real. He is immortal. Attain him, O my friend, the one goal to be attained!”