Chandogya Upanishad
Pragâpati said: 'The Self which is free from sin, free from old age, from death and grief, from hunger and thirst, which desires nothing but what it ought to desire, and imagines nothing but what it ought to imagine, that it is which we must search out, that it is which we must try to understand. He who has searched out that Self and understands it, obtains all worlds and all desires.'
The Devas (gods) and Asuras (demons) both heard these words, and said: 'Well, let us search for that Self by which, if one has searched it out, all worlds and all desires are obtained.' Thus saying Indra went from the Devas, Virokana from the Asuras, and both, without having communicated with each other, approached Pragâpati, holding fuel in their hands, as is the custom for pupils approaching their master.
They dwelt there as pupils for thirty-two years. Then Pragâpati asked them: 'For what purpose have you both dwelt here?' They replied: 'A saying of yours is being repeated, viz. "the Self which is free from sin, free from old age, from death and grief, from hunger and thirst, which desires nothing but what it ought to desire, and imagines nothing but what it ought to imagine, that it is which we must search out, that it is which we must try to understand. He who has searched out that Self and understands it, obtains all worlds and all desires." Now we both have dwelt here because we wish for that Self.' Pragâpati said to them: 'The person that is seen in the eye, that is the Self. This is what I have said. This is the immortal, the fearless, this is Brahman.' They asked: 'Sir, he who is perceived in the water, and he who is perceived in a mirror, who is he?' He replied: 'He himself indeed is seen in all these.'