Maharishi used this expression. "Jivo brahmaiva naparah." It was among those highlighted by MIU president Bevan Morris in the "Vedic Expressions used by Maharishi" continuing ed course 2 or 3 years ago. No source was cited. The translation is "the individual (ie jiva) is brahm." Brahm meaning totality. Compare to the expression: "Brahmavit brahmaiva bhavati" from the Mundaka Upanishad (3.2.9) which translates to "the knower of brahm is brahm". It all shakes out as: you are the universe. I am the universe. We are each the totality of all there is. And that's all we are!
Note: the moderator, remarked that this answer was unclear. !!!??? Hmmmm. Maharishi was Maharishi Mahesh Yogi, world famous founder of the Transcendental Meditation movement. Disciple of Swami Brahmanada Saraswati, who held the position of Shankaracharya of Jyotir math in the 1950s, maybe into the 60s. Maharishi was easily the greatest sage of Vedic science since Shankara. Came out of the Himalayas in 1958. Established a university in the USA in 1971: Maharishi International University, which recently celebrated its 50th anniversary. If you want to understand Vedic expressions, the most authoritative source is MIU. But in truth, this inquiry is much more than an intellectual thing. The experience of transcendence, which Maharishi identified as the foundational kernel of the Veda, and which he brilliantly standardized, simplified and made available for all with his Transcendental Meditation technique and related programs -- without this foundation, without the foundation of refinement of consciousness all discussion of the Veda and Vedic literature is meaningless, useless. Whether we can identify the source of "jivo brahmaiva naparah" is secondary to cognizing this principle directly ourselves. Its source is thus not from some scripture; rather, its source and meaning lie within ourselves. Yourself. No one in my admittedly limited experience as a Christian-raised Westerner, is in the same ballpark as Maharishi as a spiritual master. Thus, to figure out what and where this expression comes from, and why it matters, the questioner should look into the TM program. There are certified TM teachers around the world.