Chandogya Upanishad 8.11

'When a man being asleep, reposing, and at perfect rest sees no dreams, that is the Self, this is the immortal, the fearless, this is Brahman.' Then Indra went away satisfied in his heart. But before he had returned to the Devas, he saw this difficulty. In truth he thus does not know himself (his self) that he is I, nor does he know anything that exists. He is gone to utter annihilation. I see no good in this.

Taking fuel in his hand he went again as a pupil to Pragâpati. Pragâpati said to him: 'Maghavat, as you went away satisfied in your heart, for what purpose did you come back?' He said: 'Sir, in that way he does not know himself (his self) that he is I, nor does he know anything that exists. He is gone to utter annihilation. I see no good in this.'

'So it is indeed, Maghavat,' replied Pragâpati; 'but I shall explain him (the true Self) further to you, and nothing more than this. Live here other five years.' He lived there other five years. This made in all one hundred and one years, and therefore it is said that Indra Maghavat lived one hundred and one years as a pupil with Pragâpati. Pragâpati said to him: