There is a speech that was given at EPIC (Electronic Privacy Information Center) in 2015 that is likely to have the quote you are looking for.[0][1]
There is a short clip of his speech at the event which was reported by NBC News, in which he states the boundaries of their advertisement program at the time.[2]
EDIT: There is also this story from the Verge in 2014 which includes a lengthy quote about the advertisement program.[3]