“A work is never completed except by some accident such as weariness, satisfaction, the need to deliver, or death: for, in relation to who or what is making it, it can only be one stage in a series of inner transformations.”

"Recollection", Collected Works, vol. 1 (1972), as translated by David Paul
Variant translations:
A poem is never finished; it's always an accident that puts a stop to it — i.e. gives it to the public.
As attributed in Susan Ratcliffe, Concise Oxford Dictionary of Quotations (2011), p. 385.
A poem is never finished; it is only abandoned.
Widely quoted, this is a paraphrase of Valéry by W. H. Auden in 1965. See W. H. Auden: Collected Poems (2007), ed. Edward Mendelson, "Author's Forewords", p. xxx.
An artist never finishes a work, he merely abandons it.
A paraphrase by Aaron Copland in the essay "Creativity in America," published in Copland on Music (1944), p. 53
In the eyes of those lovers of perfection, a work is never finished — a word that for them has no sense — but abandoned; and this abandonment, whether to the flames or to the public (and which is the result of weariness or an obligation to deliver) is a kind of an accident to them, like the breaking off of a reflection, which fatigue, irritation, or something similar has made worthless.