Need help -- tangled web??

I am away from my library and I'm nothing without my reference books. Is the following correct? Not Shakespeare?

"Oh, what a tangled web we weave,
When first we practice to deceive!"
-Sir Walter Scott, Marmion 1808.

William Shakespeare got there first I'm afraid.

I can't find it in Shakespeare. Bartlett's says Scott.

I'm afraid I misinformed you. My 'of the cuff' recollection was wrong: it's not Shakespeare but Scott. While I researched I trawled with Google and it's surprising how many websites out there also believe it's Shakespeare - variously Hamlet or Macbeth.

My apologies - why do I still have a nagging doubt?