“A word to the wise is enough.”

Introductio ad prudentiam: Part II (1727), Gnomologia (1732)

Interview, The Paris Review (Summer 1956)

"The Weaver and the Worm", The New Yorker ( 11 August 1956 http://www.newyorker.com/archive/1956/08/11/1956_08_11_019_TNY_CARDS_000252308); Further Fables for Our Time (1956)
From Fables for Our Time and Further Fables for Our Time

Fragment 50, as translated in the Loeb Classics edition http://www.loebclassics.com/view/heracleitus_philospher-universe/1931/pb_LCL150.471.xml?rskey=IyhfrN&result=8
Variant translations:
Listening not to me but to reason, it is wise to agree that all is one.
Listening not to me but to the Word it is wise to agree that all things are one.
He who hears not me but the logos will say: All is one.
It is wise to hearken, not to me, but to my Word, and to confess that all things are one.
The word translated in these quotes and many others as "The Word" or "Reason", is the greek word λόγος (Logos).
Numbered fragments

Ch 6
A Canticle for Leibowitz (1959), Fiat Homo
Context: It was said that God, in order to test mankind which had become swelled with pride as in the time of Noah, had commanded the wise men of that age, among them the Blessed Leibowitz, to devise great engines of war such as had never before been upon the Earth, weapons of such might that they contained the very fires of Hell, and that God had suffered these magi to place the weapons in the hands of princes, and to say to each prince: "Only because the enemies have such a thing have we devised this for thee, in order that they may know that thou hast it also, and fear to strike. See to it, m'Lord, that thou fearest them as much as they shall now fear thee, that none may unleash this dread thing which we have wrought." But the princes, putting the words of their wise men to naught, thought each to himself: If I but strike quickly enough, and in secret, I shall destroy these others in their sleep, and there will be none to fight back; the earth shall be mine.
Such was the folly of princes, and there followed the Flame Deluge.