"[Statistics] means never having to say you're sure."
I first heard this from John Elder, and it is documented here where John presented an summary of the Symposium on the Interface conference for SIGKDD Explorations (Jun06), though I think he gave the talk initially at KDD-98.
John doesn't name who said it though, and have never heard him name the person. Maybe so many have said it, that it is just one of those anonymous quotes that is ubiquitous, but in a quick search, the only place I found it was as a reference--in the title of a talk at a Fisheries conference:
Hayes, D. B., and J.R. Bence. Managing under uncertainty, or, statistics is never having to say you’re sure. Michigan Chapter of the American Fisheries Society, East Lansing, MI. 1996.