This is a qute line I heard at the beginning of a recent episode of CSI.

Wait by the river long enough, and the corpses of your enemies will come floating past
…
But those were violent times

According to this page it’s by Sun Tzu.

This is an interesting case for search engines – it’s hard to find other pages that reference that quote, however the pages surely are out there, it’s just that the wording may be slightly different. Searching on Google/Groups finds the most accurate matches.