From the Katha Upanishad

Death said: "God made sense turn outward, man therefore looks outward, not into himself. Now and again a daring soul, desiring immortality, has looked back and found himself. "The ignorant man runs after pleasure, sinks into the entanglements of death; but the wise man, seeking the undying, does not run among things that die.
"He through whom we see, taste, smell, feel, hear, enjoy, know everything. He is that Self.
"The wise man by meditating upon the self-dependent, all-pervading Self, understands waking and sleeping and goes beyond sorrow.
"Knowing that the individual self, eater of the fruit of action, is the universal Self, maker of past and future, he knows he has nothing to fear.
"He knows that He himself born in the beginning out of meditation, before water was created, enters every heart and lives there among the elements.
"That boundless Power, source of every power, manifesting itself as life, entering every heart, living there among the elements, that is Self.
"The Fire, hidden in the fire-stick like a child in the womb, worshipped with offerings, that Fire is Self.
"He who makes the sun rise and set, to Whom all powers do homage, He that has no master, that is Self.
"That which is here, is hereafter; hereafter is here. He who thinks otherwise wanders from death to death.
"Tell the mind that there is but One; he who divides the One, wanders from death to death.
"When that Person in the heart, no bigger than a thumb, is known as maker of past and future, what more is there to fear? That is Self.
"That Person, no bigger than a thumb, burning like flame without smoke, maker of past and future, the same today and tomorrow, that is Self.
"As rain upon a mountain ridge runs down the slope, the man that has seen the shapes of Self runs after them everywhere.
"The Self of the wise man remains pure; pure water, Nachiketas, poured into pure water."