From Harry Potter and the Deathly Hallows, Chapter 15, The Goblin's Revenge:
Hermione: "Your mother can’t produce food out of thin air, no one can. Food is the first of the five Principal Exceptions to Gamp’s Law of Elemental Transfigura—"
Ron: "Oh, speak English, can’t you?"
Hermione: "It’s impossible to make good food out of nothing! You can Summon it if you know where it is, you can transform it, you can increase the quantity if you’ve already got some..."
About Gamp's Law of Elemental Tranformation:
Gamp's Law of Elemental Transfiguration is a law governing the magical world. There are five Principle Exceptions to Gamp's Law; the only one explicitly stated is food - however, there are a number of possible candidates for the remaining four.
It should be noted that while food cannot be outright created from nothing, it can be multiplied if one already has some food to multiply, it can be enlarged or the food can be summoned if one knows the approximate location and is fairly sure the food will still be there.1
In September 1997, after Ron commented that his mother could make good food out of thin air, Hermione stated that this was impossible, as food is one of the five Principle Exemptions to this law, but this only irritated Ron, who wanted her to "speak English", but was not satisfied by her explanation.
On 1 May, 1998, after Neville Longbottom noted that the one thing that the Room of Requirement did not seem to be able to provide was food, Ron commented that food was one of the five Principle Exceptions to this law, to "general astonishment".