I presume that you're referring to Wiles' statement to the effect that, "Proving Fermat's Last Theorem is probably the biggest thing that I'll ever do." As a young student, it made a big impression. Eighteen years later, as a professional scientist, it's even more poignant.
Found it in a NOVA transcript [1]:
"At the beginning of September, I was sitting here at this desk, when suddenly, totally unexpectedly, I had this incredible revelation. It was the most—the most important moment of my working life. Nothing I ever do again will. . . I'm sorry."
[1] http://www.pbs.org/wgbh/nova/transcripts/2414proof.html