Full Idea
If the practice of virtue were to become a habit the subject would suffer loss to that freedom in adopting his maxims which distinguishes an action done from duty.
Gist of Idea
If virtue becomes a habit, that is a loss of the freedom needed for adopting maxims
Source
Immanuel Kant (Metaphysics of Morals II:Doctrine of Virtue [1797], 409 Intro XVI)
Book Reference
Kant,Immanuel: 'The Metaphysics of Morals', ed/tr. Gregor,Mary [CUP 1991], p.210
A Reaction
Looks like a misunderstanding of Aristotle, who always promotes the role of 'phronesis' [practical reason], and never advocates unthinking virtuous habits. I think Aristotle would ask how you select your maxim, if you lack the virtues.