This famous quote from Albert Einstein, after research, doesn't have any origins (No year, no location/event, etc.). How would you go about citing it in APA format?
If there's no source for the quote, it's very likely misattributed. A lot of quotes get falsely attributed to Einstein, probably to give them a little more weight. Proper citation means that you don't propagate this false information, and in fact, do your best to set the record straight.
The earliest references on Google books are from around 1945 and attribute the saying to "Anonymous".
I'd attribute the quote to Anonymous and add a footnote saying something like:
This saying is often credited to Albert Einstein, but there is no credible source for this attribution.