Quoted by Ernst G. Straus, who was Einstein's assistant from 1944 to 1948, in Carl Seelig, Helle Zeit—Dunkle Zeit (Europa Verlag, Zurich, 1956), p. 72
Was mich eigentlich interessiert, ist, ob Gott die Welt hätte anders machen können; das heisst, ob die Forderung der logischen Einfachheit überhaupt eine Freiheit lässt.
Original text (German)
Notes from Wikiquote
As translated in Max Jammer, Einstein and Religion (Princeton University Press, 1999), p. 124 | What I'm really interested in is whether God could have made the world in a different way; that is, whether the necessity of logical simplicity leaves any freedom at all. | As translated in Gerald Holton, The Scientific Imagination: Case Studies (Cambridge University Press, 1978), p. xii