Plato was an excercise freak

All over the net, exercise fans are quoting Plato:

"Lack of activity destroys the good condition of every human being, while movement and methodical physical exercise save it and preserve it."

Where exactly is this from? I've been trying to decipher in which text Plato supports exercise, but with no joy so far. Is it apocryphal? Is it bullshit? Where's it from, tell me so I can check it! Any ideas?