About the use of magic... Going Postal I

"Oh, you could do it all by magic, you certainly could. You could wave a wand and get twinkly stars and a fresh-baked loaf. You could make fish jump out of the sea ready cooked. And then, somewhere, somehow, magic would present its bill, which was always more than you could afford.
That's why it was left to wizards, who knew how to handle it safely. Not doing any magic at all was the chief task of wizards - not 'not doing magic' because they couldn't do magic, but not doing magic when they could do and didn't. Any ignorant fool can fail to turn someone else into a frog. You have to be clever to refrain from doing it when you know how easy it is. There were places in the world commemorating those times when wizards hadn't been quite as clever as that, and on many of them the grass would never grow again."

in Going Postal by Terry Pratchett

Ring a bell somewhere?