Source, in full
This quote does appear in Einstein's 1940 essay "The Fundaments of Physics" which can be found in his book Out of My Later Years (1950), but Einstein does not claim credit for it, instead calling it "Lessing's fine saying".
More quotes by Albert Einstein
A happy man is too satisfied with the present to dwell too much on the future.
Try to become not a man of success, but try rather to become a man of value.
Life is like riding a bicycle. To keep your balance you must keep moving.
Attribution
Quote text and notes from Wikiquote: Albert Einstein (page history), available under CC BY-SA 4.0. Source quality and labels are our own summary of what Wikiquote records.