“As a general rule, all that has been hitherto advanced respecting the nature of this deity, must be understood to refer to his properties: for the nature of the god is not one thing, and his influence another: and truly, besides these two, his energy a third thing: seeing that all things which he wills, these he is, he can, and he works. For neither doth he will that which he is not; nor is he without strength to do that which he wills; nor doth he will that which he cannot effect. Now this is very different in the case of men, for theirs is a double nature mixed up in one, that of soul and body; the former divine, the latter full of darkness and obscurity: hence naturally arise warfare and discord between the two.”

Source: Self-Reliance

This attribution occurs in chapter 13 (Ioan. Graphei, 1532, p. 494) http://books.google.com/books?id=rs47AAAAcAAJ&pg=PA494 of the Christian church father's Lactantius's De Ira Dei (c. 318):
"God," he [Epicurus] says, "either wants to eliminate bad things and cannot,
or can but does not want to,
or neither wishes to nor can,
or both wants to and can.
If he wants to and cannot, then he is weak and this does not apply to god.
If he can but does not want to, then he is spiteful which is equally foreign to god’'s nature.
If he neither wants to nor can, he is both weak and spiteful, and so not a god.
If he wants to and can, which is the only thing fitting for a god, where then do bad things come from? Or why does he not eliminate them?"
Lactantius, On the Anger of God, 13.19
Charles Bray, in his 1863 The Philosophy of Necessity: Or, Natural Law as Applicable to Moral, Mental, and Social Science quotes Epicurus without citation as saying a variant of the above statement (p. 41) http://books.google.com/books?id=BebVAAAAMAAJ&pg=PA41 (with "is not omnipotent" for "is impotent"). This quote appeared in "On the proofs of the existence of God: a lecture and answer questions" http://www.atheism.ru/old/KryAth2.html (1960) by professor Kryvelev I.A. (Крывелёв И.А. О доказательствах бытия божия: лекция и ответы на вопросы. М., 1960). And N. A. Nicholson, in his 1864 Philosophical Papers (p. 40), attributes "the famous questions" to Epicurus, using the wording used earlier by Hume (with "is he" for "he is") http://books.google.com/books?id=ZMsGAAAAcAAJ&pg=PA40. Hume's statement occurs in Book X (p. 186) http://books.google.com/books?id=E7dbAAAAQAAJ&pg=PA186 of his renowned Dialogues Concerning Natural Religion, published posthumously in 1779. The character Philo precedes the statement with "Epicurus's old questions are yet unanswered.…". Hume is following the enormously influential Dictionnaire Historique et Critique (1697–1702) of Pierre Bayle, which quotes Lactantius attributing the questions to Epicurus (Desoer, 1820, p. 479) http://books.google.com/books?id=QwwZAAAAYAAJ&pg=PA479.
There has also arisen a further disputed extension, for which there has been found no published source prior to The Heretic's Handbook of Quotations: Cutting Comments on Burning Issues (1992) by Charles Bufe, p. 186: "Is he neither able nor willing? Then why call him God?"
Disputed

Herbert N. Casson cited in: Forbes magazine (1950) The Forbes scrapbook of Thoughts on the business of life. p. 158
1950s and later

The Spiritual Espousals (c. 1340)

Summations, Chapter 51
Context: The Lord that sat stately in rest and in peace, I understood that He is God. The Servant that stood afore the Lord, I understood that it was shewed for Adam: that is to say, one man was shewed, that time, and his falling, to make it thereby understood how God beholdeth All-Man and his falling. For in the sight of God all man is one man, and one man is all man. This man was hurt in his might and made full feeble; and he was stunned in his understanding so that he turned from the beholding of his Lord. But his will was kept whole in God’s sight; — for his will I saw our Lord commend and approve. But himself was letted and blinded from the knowing of this will; and this is to him great sorrow and grievous distress: for neither doth he see clearly his loving Lord, which is to him full meek and mild, nor doth he see truly what himself is in the sight of his loving Lord. And well I wot when these two are wisely and truly seen, we shall get rest and peace here in part, and the fulness of the bliss of Heaven, by His plenteous grace.
And this was a beginning of teaching which I saw in the same time, whereby I might come to know in what manner He beholdeth us in our sin. And then I saw that only Pain blameth and punisheth, and our courteous Lord comforteth and sorroweth; and ever He is to the soul in glad Cheer, loving, and longing to bring us to His bliss.

Summations, Chapter 62

Source: Quotes from secondary sources, Dictionary of Burning Words of Brilliant Writers, 1895, P. 230.

The Satanic Bible (1969)

Paper communicated to Frederic Farrar (1854) Æt. 23, as quoted in Lewis Campbell, William Garnett, The Life of James Clerk Maxwell: With Selections from His Correspondence and Occasional Writings (1884) pp. 144-145, https://books.google.com/books?id=B7gEAAAAYAAJ&pg=PA144 and in Richard Glazebrook, James Clerk Maxwell and Modern Physics (1896) pp. 39-40. https://books.google.com/books?id=hbcEAAAAYAAJ&pg=PA39