The reason why the Cardinal [Mazarin] deferred so long to grant the favors he had promised, was because he was persuaded that hope was much more capable of keeping men to their duty than gratitude.

[La raison pourquoi le Cardinal différoit tant à accorder les grâces qu’il avoit promises, c’est qu’il étoit persuadé que l’espérance est bien plus capable de retenir les hommes dans le devoir que non pas la reconnoissance.]

Jean Racine (1639-1699) French dramatist

Fragments Historiques [tr. Bund/Friswell (1871)]
    (Source)

(Source (French)).

This phrase is quoted by commenters on La Rochefoucauld in conjunction with his Maxim 38.