I think it's acceptable to simply quote him without giving full source so long as you're demonstrating that you know how to reference in the rest of your paper. I'd use the 'attributed to' method too. That quote is used a lot in academic papers and doesn't ever seem to be fully referenced which may mean it's apocryphal anyway.
As an example, it's used well here:
http://www.ccsenet.org/...php/ass/article/download/9389/7128
Life, don't talk to me about life - Marvin the Paranoid Android
"Science adjusts it's views based on what's observed.
Faith is the denial of observation so that Belief can be preserved."
- Tim Minchin, in his beat poem, Storm.