Einstein is one of those major iconic figures to whom many statements become attributed; unsourced attributions to him should usually be treated with some skepticism, and often a great deal of it. The only source of knowledge is experience. Commonly attributed, google books returns over 600 phrase matches but I cannot find the original source amongst these results. Appears to be a restatement of an actual quote from The Critique of Pure Reason by Immanuel Kant.