The Aitareya Upanishad belongs to the Rig Veda and is part of the Aitareya Aranyaka.
This Upanishad is for those who have turned away from the world, whose minds are free and collected, and who are ready to acquire the knowledge of the Supreme Reality that Brahman is all pervading. He is present in all creatures whether small or big, moving or non-moving. The basis of the entire creation is Brahman the Pure Consciousness.
This Upanishad concludes by saying that a person, who realizes his oneness with the Pure Consciousness, becomes immortal.