In Carl Seelig's Albert Einstein: A Documentary Biography (1956), Seelig reports that Einstein said this to James Franck, p. 71 http://books.google.com/books?id=VCbPAAAAMAAJ&q=%22how+it+happened%22#search_anchor.
I sometimes ask myself how did it come that I was the one to develop the theory of relativity. The reason, I think, is that a normal adult never stops to think about problems of space and time. But my intellectual development was retarded, as a result of which I began to wonder about space and time only when I had already grown up. Naturally, I could go deeper into the problem than a child with normal abilities.
Variant translation which appears in Einstein: The Life and Times by Ronald W. Clark (1971), p. 27 http://books.google.com/books?id=6IKVA0lY6MAC&lpg=PP1&pg=PA27#v=onepage&q&f=false
Attributed in posthumous publications