It was from the Leigh book which accidentally leaked the password. Purportedly, it was said in a meeting with people who were largely friendly towards him at the time.
Further, additional dumps were made unredacted after the fact, and some wikileaker's claimed that it was at Assange's sole bidding. [1] Furthermore, they also intimated that it was unlikely the original leak was anything but intentional, given the password which was used, hints that were given, and the fact that they had never done that sort of thing before. Remember 100,000 cables had accidentally released by the Guardian in Feb. '11, but another 250k were released unredacted in Sept. '11.
I'm not sure to trust anyone, but I'm not putting Assange on a pedestal in this case, and certainly I don't think such a statement would be outside of his character based on the many other interviews and writings of his. Of course, he does deny saying it[2], so take everything with a grain of salt.
[1] http://www.guardian.co.uk/media/2011/sep/02/wikileaks-publis...
[2] http://www.itproportal.com/2011/05/25/assange-denies-informa...
EDIT: Just to make a point, and not be revisionist, my statement is largely incorrect about timing and numbers. My brain was temporarily jumbled digging through the articles. Guardian leaked the password, Der Spiegel added info on the file locations (purported to be planted by wikileaks in the linked article), and then wikileaks dumped it all in the open.
