Whoops! Wrong quote. But I think that's brilliant. Must be from The Theory of Moral Sentiments, but I got it from BrainyQuote and they never footnote anything.
Here's the one I meant:
[An individual is] led by an invisible hand to promote an end which was no part of his intention. Nor is it always the worse for the society that it was not part of it. By pursuing his own interest he frequently promotes that of the society more effectually than when he really intends to promote it. I have never known much good done by those who affected to trade for the public good. It is an affectation, indeed, not very common among merchants, and very few words need be employed in dissuading them from it.
People who quote that (including me) like this one, too:
It is not from the benevolence of the butcher, the brewer or the baker, that we expect our dinner, but from their regard to their own self interest. We address ourselves, not to their humanity but to their self-love, and never talk to them of our own necessities but of their advantages.
That Wikipedia article is well worth reading.