Source, in full
This quote has been attributed to Epictetus since around 2016. (e.g. in this thephilosophyofeverything blog.) Sometimes appears in an extended form, e.g. by The Daily Stoic: "You become what you give your attention to. If you yourself don't choose what thoughts and images you expose yourself to, someone else will." These quotations originate from the 1995 book: The Art of Living: the classic manual on virtue, happiness, and effectiveness, pages 52–53, by Sharon Lebell.