Source
The Story of Philosophy
Will Durant's popular survey of Western philosophers from Plato to Dewey, published in 1926. Its chapter on Aristotle contains the line 'We are what we repeatedly do', Durant's own summary, which is often quoted as Aristotle's words.
References
Quotes (1)
“We are what we repeatedly do. Excellence, then, is not an act, but a habit.”
— Will Durant (often misattributed to Aristotle), The Story of Philosophy (1926), summarising Aristotle; listed as misattributed on Wikiquote