Source
Cratylus
A dialogue by Plato on whether names belong to things by nature or by convention. In it Socrates reports Heraclitus's saying that you cannot step into the same river twice.
References
Quotes (1)
“Heracleitus is supposed to say that all things are in motion and nothing at rest; he compares them to the stream of a river, and says that you cannot go into the same water twice.”
— Heraclitus (as reported by Socrates in Plato), Plato, Cratylus 402a, tr. Benjamin Jowett