The man who does not recompense his benefactor, when he has it in his power, and when his benefactor needs his assistance, is, no doubt, guilty of the blackest ingratitude. The heart of every impartial spectator rejects all fellow-feeling with the selfishness of his motives, and he is the proper object of the highest disapprobation.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“It may disappoint of the good which might reasonably have been expected, and upon that account it may justly excite dislike and disapprobation: it cannot, however, provoke any resentment which mankind will go along with. The man who does not recompense his benefactor, when he has it in his power, and when his benefactor needs his assistance, is, no doubt, guilty of the blackest ingratitude. The heart of every impartial spectator rejects all fellow-feeling with the selfishness of his motives, and he is the proper object of the highest disapprobation. But still he does no positive hurt to any body. He only does not do that good which in propriety he ought to have done. He is the object of hatred, a passion which is naturally excited by impropriety of sentiment and behaviour;” source