You can't step twice in the same river,
for the water into which you first stepped has flowed on.
This statement has been translated variously, as the Fragments of Heraclitus quoted here indicate. Another variant, found here, is, "You could not step twice into the same river; for other waters are ever flowing on to you." Also see Heraclitus: The Complete Fragments, comprising translations and commentary, with Greek text, from William Harris, professor emeritus, Middlebury College; specifically see the section "This Paradoxical Universe".
Heraclitus (c. 535 - c. 475 BCE), Greek Philosopher
Heraclitus at Internet Encyclopedia of Philosophy