"I have repeatedly said the only time to use Social Security money is in times of war, times of recession, or times of severe emergency. And I mean that. I mean that." (Transcript of Remarks by Presidents Bush and Fox on Departure to Toledo, Ohio -- U.S. Newswire, Inc, September 6, 2001)
Sorry, I was incorrect about it being specifically mentioned in the general-election debates. I'm reviewing transcripts of the primary debates to see if it appeared there...but I know he spoke of it before September 6'01.