Kiki - you might be right, I have also seen it attributed to Mozart, so I did a bit of digging to try and find the actual source. As I found a reasonably reputable source for the Beethoven quote, I went with that:
"Beethoven, meeting Paer after a performance of the latter's opera Eleanora, said to him, " I like your opera : I think I will set it to music." That was perhaps not very nice, at the moment, for Paer ; but as the result was Fidelio..."
from "A Musical Moteley" by ERNEST NEWMAN
http://www.archive.org/stream/musica...muoft_djvu.txt
Do you have a source for the Mozart quote? Maybe Beethoven couldn't resist stealing it...