I don't think that's an exact quote...
If I remember correctly, it's in AGATB when Felicity and the others are about to kill the deer... But I'm not even close to positive on that. =/
I think this is what it really is:
"That is how fires start. With a spark. And I see the spark catching the wind."
Edit: It might also be at the end of Rebel Angels when Gemma and Kartik have just decided to make the alliance. Again, not positive.