We are what we repeatedly do. Excellence, therefore, is not an act but a habit.

Aristotle (384-322 BC) Greek philosopher

Nicomachean Ethics [Ἠθικὰ Νικομάχεια] (c. 325 BC) (paraphrase)
    (Source)

Variants:

• "We are what we repeatedly do. Excellence, then, is not an act, but a habit."
• "We are what we repeatedly do, therefore excellence is not an act, but a habit."

Not actually Aristotle, but a summary by  Will Durant, The Story of Philosophy: The Lives and Opinions of the World's Greatest Philosophers (1926), ch. 2 "Aristotle and Greek Science," sec. 7 "Ethics and the Nature of Happiness" (1926):

Excellence is an art won by training and habituation: we do not act rightly because we have virtue or excellence, but we rather have these because we have acted rightly; "these virtues are formed in man by his doing the actions"; we are what we repeatedly do. Excellence, then, is not an act but a habit: "the good of man is a working of the soul in the way of excellence in a complete life ... for as it is not one swallow or one fine day that makes a spring, so it is not one day or a short time that makes a man blessed and happy."

The quoted phrases are from the Nicomachean Ethics, Book 2, ch. 4; Book 1, ch. 7.