Yes, but its a paraphrase, from memory.
I just looked up the exact quote:
Peter Guillam: So Karla's fireproof. He can't be bought, and he can't be beaten.
George Smiley: NOT fireproof! Because's he's a fanatic! I may have acted like a soft dolt, the very archetype of a flabby Western liberal but I'd rather be my kind of fool than his. One day that lack of moderation will be Karla's downfall.