You're right to ask for citation. Put simply, there is no definitive proof that the primary was rigged per se, but there were consistent differences between the exit polling & the voting results. Almost all of the differences were in Secretary Clinton's favor, and these discrepancies were not found in the Republican primaries, nor were they found in the '08 primaries.

http://www.snopes.com/stanford-study-proves-election-fraud-t...