Whether we believe the Greek poet, “it is sometimes even pleasant to be mad”, or Plato, “he who is master of himself has knocked in vain at the doors of poetry”; or Aristotle, “no great genius was without a mixture of insanity”; the mind cannot express anything lofty and above the ordinary unless inspired. When it despises the common and the customary, and with sacred inspiration rises higher, then at length it sings something grander than that which can come from mortal lips. It cannot attain anything sublime and lofty so long as it is sane: it must depart from the customary, swing itself aloft, take the bit in its teeth, carry away its rider and bear him to a height whither he would have feared to ascend alone.

[Nam sive Graeco poetae credimus ‘aliquando et insanire iucundum est,’ sive Platoni ‘frustra poeticas fores compos sui pepulit,’ sive Aristoteli ‘nullum magnum ingenium sine mixtura dementiae fuit’: non potest grande aliquid et super ceteros loqui nisi mota mens. Cum vulgaria et solita contempsit instinctuque sacro surrexit excelsior, tunc demum aliquid cecinit grandius ore mortali. Non potest sublime quicquam et in arduo positum contingere, quam diu apud se est; desciscat oportet a solito et efferatur et mordeat frenos et rectorem rapiat suum eoque ferat, quo per se timuisset escendere.]

Seneca the Younger (c. 4 BC-AD 65) Roman statesman, philosopher, playwright [Lucius Annaeus Seneca]

Moral Essays, “On Tranquility of Mind [De Tranquillitate Animi],” 17.10 [tr. Langsdorf (1900)]
    (Source)

(Source (Latin)). More on the Aristotle (mis)quote here.