We are what we repeatedly do. Excellence, then, is not an act but a habit.
Wrongly attributed
This is Will Durant, not Aristotle, and Durant is not translating -- he is summarising the Nicomachean Ethics in his own words and marking the difference by dropping the quotation marks he uses on either side of it. The nearest thing in Aristotle is that virtues are formed in a man by his doing the actions, which Durant does put in quotation marks in the same sentence. The memorable half is the half Durant wrote. It usually circulates with a therefore inserted and the excellence turned into a moral instruction, neither of which is in the book.