Link to the source (http://www.nngroup.com/articles/scrolling-and-attention/) is in the comments.

However, as the author admits in the comments (after being corrected by a reader), he has interpreted the results of the source incorrectly. The cited report actually only says that "Web users spend 80% of their time looking at information above the page fold. Although users do scroll, they allocate only 20% of their attention below the fold."