Sure, it's from the Washington Post's reporting: http://www.washingtonpost.com/wp-srv/special/politics/prism-...

Search for 51.

I see where it says in the caption "The supervisor must endorse the analyst's "reasonable belief," defined as 51 percent confidence, that the specified target is a foreign national who is overseas at the time of collection." But that's a caption written by the Post. What I don't see is any support for that statement in the actual slide itself, nor any of the other slides on that page.