This "quote" is probably bogus. When I was at lunch with Knuth once (brag, brag), someone asked him where he said that. Knuth said he couldn't recall saying it and he doubted that he ever said it, because he would not attempt to prove code without also testing it. Then he said it might be a paraphrase (not a quote) from an analysis he made of an assembly language program where he did not have hardware with that instruction set available to him.