Will Durant
The Story of Philosophy, chapter 2, 1926
The line is a fair summary of Aristotle's idea that character is built by practice. It is Durant's, though, and it is a good illustration of how a paraphrase gets promoted to a quote.
the myth (Aristotle): Durant was summing up Aristotle's ethics in his own words. The sentence is often printed under Aristotle's name, but you won't find it in his texts.
Source: Wikiquote