Shandilya Upanishad of Atharvaveda, Chapter II

Then the Brahmarṣi Śāṇḍilya not obtaining the knowledge of Brahman in the four Vedas, approached the Lord Atharvan and asked him: "What is it? Teach me the science of Brahman by which I shall obtain that which is most excellent."

Atharvan replied: "O Śāṇḍilya, Brahman is satya, vijñāna and ananta in which all this (world) is interwoven, warp-wise and woof-wise, from which all originated and into which all are absorbed, and which being known makes everything else known. It is without hands and feet, without eyes and ears, without tongue or without body, and is unreachable and undefinable. From which, vāk (speech) and mind return, being unable to obtain (or reach) It. It is to be cognised by jñāna and yoga.^[1] From which, prajñā of old sprang. That which is one and non-dual, that which pervades everything like ākāś, which is extremely subtle, without a blemish, actionless, sat (be-ness) only, the essence of the bliss of consciousness, beneficent, calm and immortal and which is beyond. That is Brahman. Thou art That. Know That by wisdom. He who is the one, the shining, the giver of the power of Ātmā, the omniscient, the lord of all, and the inner soul of all beings, who lives in all beings, who is hidden in all beings and the source of all beings, who is reachable only through yoga and who creates, supports and destroys everything—He is Ātmā. Know the several worlds in the Ātma. Do not grieve, O knower of Ātmā, thou shalt reach the end of pains."