It's said by the dragon in the first film, though in a slightly different phrasing. The full quote is:
"Dragon: You're a little... crunchy. I like my men like I like my dragon eggs...soft and gooey.
Donkey: Oh, no, she eats people!
Dragon: Well, I haven't eaten anyone in weeks. I just have a little munchie, that's all.
Donkey: Well, why don't you just have a cookie?
Dragon: Why not? Maybe I will. ... But not until I finish this one.
Shrek: You... you're not gonna eat him, are you?
Dragon: Oh, relax, little fella. It's not like I'm gonna eat you. ... Now, where were we? Oh yeah, don't meddle in the affairs of dragons, for you are crunchy and taste good with ketchup."