Sorry, I can't find it anymore. I got it from the youtube channel by Sam Vaknin. He is a convicted coin man with NPD, so take everything he says with a grain of salt. That having said, his work tend to lack the sort of victim pandering you see in other narcissism "experts".

EDIT: there are a number of other studies on google scholar linking ADHD to psychopathy (in prison populations), just not the one with the fMRI scans.