Tat Tvam Asi
Three words. Nine repetitions.
In the sixth chapter of the Chandogya Upanishad, Uddalaka Aruni teaches his son Shvetaketu, who has returned from twelve years of study confident that he knows. The father asks whether he has learned that by which the unheard becomes heard, the unthought becomes thought. The son has not.
What follows is a sequence of ordinary illustrations — clay and pots, salt dissolved in water, rivers reaching the sea, a banyan seed cut open to reveal nothing inside. After each, the same instruction: that subtle essence is the truth of all that exists. Tat tvam asi, Shvetaketu.
That which is the subtle essence — this whole world has that as its Self. That is the truth. That is the Self. That thou art.
It is worth noting what is not said. Not "you will become that." Not "that will be given to you." The verb is present tense. Nothing is promised because nothing is missing — only unrecognised.
Why nine times?
Because hearing it once produces agreement, and agreement is not knowledge. The repetition is the method: the same conclusion approached from nine directions, until the intellect cannot find an exit.