After some more searching and a pointer on Straight Dope, I think I've found it in Book 7 of the Histories when Artabanus is trying to dissuade Xerxes from launching his ill-fated war against the Greeks, where it is, as one would expect from Jaynes's paraphrase, different:

"1 So do not plan to run the risk of any such danger when there is no need for it. Listen to me instead: for now dismiss this assembly; consider the matter by yourself and, whenever you so please, declare what seems best to you. 2 A well-laid plan is always to my mind most profitable; even if it is thwarted later, the plan was no less good, and it is only chance that has baffled the design; but if fortune favor one who has planned poorly, then he has gotten only a prize of chance, and his plan was no less bad."

Or in another translation:

"Think then no more of incurring so great a danger when no need presses, but follow the advice I tender. Break up this meeting, and when thou hast well considered the matter with thyself, and settled what thou wilt do, declare to us thy resolve. I know not of aught in the world that so profits a man as taking good counsel with himself; for even if things fall out against one's hopes, still one has counselled well, though fortune has made the counsel of none effect: whereas if a man counsels ill and luck follows, he has gotten a windfall, but his counsel is none the less silly."