This quote has variously been attributed (or misattributed) to Einstein, Ben Franklin, Mark Twain, and Bill Wilson. I've heard that it appears in Einstein's book, Letters to Solovine, but I've never seen a page reference. It definitely appears in print in the 1983 novel Sudden Death by Rita Mae Brown (pg. 68), but she could have very well been borrowing it. Sorry, but there seems to little proof positive to attribute it to Einstein or anyone else for that matter.
Hope this helps!