"Morals excite passions, and produce or prevent actions. Reason of itself is utterly impotent in this particular. The rules of morality, therefore, are not conclusions of our reason." - David Hume

Note about the selection of this quote: While I am not inclined towards the position that reason is (and ought to be) slave to the passions, I considered this a good quote on the topic of rationality because it concisely presents one of the most fundamental challenges for rationalism as such.