“Kill your darlings,”
Stephen King Hemingway Faulkner Arthur Quiller-Couch said to young authors, in his 1914 lecture “On Style”:
Style, for example, is not—can never be—extraneous Ornament. You remember, may be, the Persian lover whom I quoted to you out of Newman: how to convey his passion he sought a professional letter-writer and purchased a vocabulary charged with ornament, wherewith to attract the fair one as with a basket of jewels. Well, in this extraneous, professional, purchased ornamentation, you have something which Style is not: and if you here require a practical rule of me, I will present you with this: ‘Whenever you feel an impulse to perpetrate a piece of exceptionally fine writing, obey it—whole-heartedly—and delete it before sending your manuscript to press. Murder your darlings.’ [Emphasis not mine.]
I found it goes back a little further than that. In 1759, Edward Young published a letter to Samuel Richardson, an extremely popular British novelist of the 18th century, in “Conjectures on Original Composition” (excerpted in Critical Theory Since Plato, 3rd edition, p. 329). It said:
Wit, indeed, however brilliant, should not be permitted to gaze self-enamored on its useless charms, in that fountain of fame (if so I may call the press), if beauty is all that it has to boast; but, like the first Brutus , it should sacrifice its most darling offspring to the sacred interests of virtue, and real service of mankind.
 Lucius Junius Brutus, founder of the Roman Republic, who put his sons to death for participating in a political conspiracy