On the importance of mindfulness of thoughts (attributed to Aristotle)

“It is the mark of an educated mind to be able to entertain a thought without accepting it.” 
This is attributed to  Aristotle, Metaphysics  ihn many places on the internet.
However, I have scoured the Metaphysics and  cannot find it! 
I like the quote anyway, but perhaps we need in the internet age to also add
"It is the mark of an educated mind to be able to read a quotation on the internet without accepting it as authoritative unless we have checked the original source". Not quite as pithy!