There is also this line from Taittriya Aranyaka chapter 10 -
VIII-1: The Supreme Being, Indra, who is the most excellent Pranava taught in the Vedas, who ensouls the entire universe, who leads the collection of Vedic utterances in Gayatri and other metres standing in their beginning, who is capable of being attained by the worshippers and who is the first in the causal link, taught the contemplative sages the sacred wisdom of the Upanishad, Himself being the subject-matter of them, in order to strengthen them with the power of knowledge. I salute the gods for removing the obstacles in my path to illumination. For the same I also reverence the Manes. The triple regions of Bhuh, Bhuvah and Suvah and the entire Veda are comprised in Om.
See also Kaushitaki Brahmana Upanishad 3.1-2
1. Pratardana, forsooth, the son of Divodasa (king of Kasi), came by means of fighting and strength to the beloved abode of Indra. Indra said to him 'Pratardana, let me give you a boon to choose.' And Pratardana answered: 'Do you yourself choose that boon for me which you deem most beneficial for a man.' Indra said to him: 'No one who chooses, chooses for another; choose thyself.' Then Pratardana replied : 'Then that boon to choose is no boon for me.' Then, however, Indra did not swerve from the truth, for Indra is truth. Indra said to him: 'Know me only; that is what I deem most beneficial for man, that he should know me. I slew the three-headed son of Tvashtri; I delivered the Arunmukhas, the devotees, to the wolves (salavrika); breaking many treaties, I killed the people of Prahlada in heaven, the people of Puloma in the sky, the people of Kalakanga on earth. And not one hair of me was harmed there. And he who knows me thus, by no deed of his is his life harmed, not by the murder of his mother, not by the murder of his father, not by theft, not by the killing of a Brahman. If he is going to commit a sin, the bloom I does not depart from his face! not depart from his face! 2. Indra said: 'I am prana, meditate on me as the conscious self (pragnatman), as life, as immortality. Life is prana, prana is life. Immortality is prana, prana is immortality. As long as prana dwells in this body, so long surely there is life. By prana he obtains immortality in the other world, by knowledge true conception. He who meditates on me as life and immortality, gains his full life in this world, and obtains in the Svarga world immortality and indestructibility.'
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