“Never hesitate to imitate another writer - every person learning a craft or an art needs models. Eventually you'll find your own voice and will shed the skin of the writer you imitated.”

L’écrivain original n’est pas celui qui n’imite personne, mais celui que personne ne peut imiter.
The Oxford Dictionary of Quotations (1979) 3rd edition
Variant translations:
The original style is not the style which never borrows of any one, but that which no other person is capable of reproducing.
As translated by Charles I. White (1856) Part 2, Book 1, Chapter 3
An original writer is not one who imitates nobody, but one whom nobody can imitate.
Bartlett's Familiar Quotations (1980) 15th edition.
Le génie du Christianisme (1802)

Live at the Apollo (November 26, 2007)

Afterword to The Dud Avocado (2006)
Context: The Big Personalities weighed in. Soon after its publication Irwin Shaw wrote to me praising it. Terry Southern, calling me "Miss Smarts," said I was "a perfect darling." Gore Vidal phoned one morning saying, "You’ve got the one thing a writer needs: You’ve got your own voice. Now go." Ernest Hemingway said to me, "I liked your book. I liked the way your characters all speak differently." And then added, "My characters all sound the same because I never listen." All this, and heaven too. Laurence Olivier told me that now that my book was making a lot of money we could elope and I could support us. The Financial Times ran an item which read, "Such and such stock: No dud avocado." Groucho Marx wrote me, "I had to tell someone how much I enjoyed The Dud Avocado.… If this was actually your life, I don’t know how the hell you got through it." When people ask me how autobiographical the book is I say, all the impulsive, outrageous things my heroine does, I did. All the sensible things she did, I made up.

Source: Self-Reliance and Other Essays

The Decay of Lying (1889)

Source: The Pilgrimage