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
