“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 [1], it should sacrifice its most darling offspring to the sacred interests of virtue, and real service of mankind.

[1]  Lucius Junius Brutus, founder of the Roman Republic, who put his sons to death for participating in a political conspiracy