“Wherefore not without cause has one of your own followers asked, "If God is, whence come evil things? If He is not, whence come good?"”

Upon The Mother Of The Gods (c. 362-363)
Context: To what purpose, pray, exist all these things that be born? Whence come male and female? Whence the difference in kind of all things that be, amongst visible species, unless there be certain pre-existing and previously established Reasons and Causes subsisting beforehand, in the nature of a pattern? With regard to which, though we are dull of sight, yet let us strive to clear away the mist from the eyes of the soul.

Gotthold Ephraim Lessing, as quoted in Dictionary of Quotations from Ancient and Modern English and Foreign Sources (1899) by James Wood, p. 61; usually attributed to Doris Lessing in the form: "Man — who is he? Too bad, to be the work of God: Too good for the work of chance!"
Misattributed

Der Mensch, wo ist er her?
Zu schlecht für einen Gott, zu gut fürs Ungefähr.
As quoted in Dictionary of Quotations from Ancient and Modern English and Foreign Sources (1899) by James Wood, p. 61
Variant: Man — who is he? Too bad to be the work of God; Too good for the work of chance!

Plato, Phaedo

Quoted in Ruysbroeck the Admirable (1925) by Alfred Wautier d'Aygalliers and Fred Rothwell, p. 175

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

The Theosophical Writings of Annie Besant http://books.google.co.in/books?id=eIrD4GQFCPIC&pg=PT437

As quoted in "Voices of the New Time" as translated by C. C. Shackford in The Radical Vol. 7 (1870), p. 329

Philo to Cleanthes, Part X
Dialogues Concerning Natural Religion (1779)
Context: And is it possible, CLEANTHES, said PHILO, that after all these reflections, and infinitely more, which might be suggested, you can still persevere in your Anthropomorphism, and assert the moral attributes of the Deity, his justice, benevolence, mercy, and rectitude, to be of the same nature with these virtues in human creatures? His power we allow is infinite: whatever he wills is executed: but neither man nor any other animal is happy: therefore he does not will their happiness. His wisdom is infinite: He is never mistaken in choosing the means to any end: But the course of Nature tends not to human or animal felicity: therefore it is not established for that purpose. Through the whole compass of human knowledge, there are no inferences more certain and infallible than these. In what respect, then, do his benevolence and mercy resemble the benevolence and mercy of men? EPICURUS's old questions are yet unanswered. Is he willing to prevent evil, but not able? then is he impotent. Is he able, but not willing? then is he malevolent. Is he both able and willing? whence then is evil?