FALSE ALERT.. First, this is a totally exaggerated, misleading FAR REACHING claim.
Let's look at the good first: Oranges are an excellent source of vitamin C (but not as much as we need) they're also a very good source of dietary fiber, vitamin B1, pantothenic acid, and folate as well as vitamin A, calcium, copper, and potassium, BUT to address the claims of hydration and prevention of muscles getting sore, that's just bunk. Especially to go as far as "keeping you hydrated" and "preventing your muscles from getting sore". The only thing that's at all helpful for hydration is the little bit of potassium citrate that may (MAY if you're abnormally low) help prevent some cramping by holding water intramuscular, but again, it's not doing much. All the sugar right before a workout is completely counter productive and the other vitamins, minerals etc., although good for you are in very small amounts overall.
The claim of "preventing your muscles from getting sore"... Well that's even more far reaching. Oranges don't prevent muscles tears (why you're sore after working out) or help heal them either. They also didn't specify what "workout" they mean.. Cardio? Strength? I'm assuming strength training because of the reference to muscle soreness, but either way it's bunk. My advice. Eat the orange with your meal and get the benefit without the slap in the face from sugar and drink adequate amounts of water before, during and after your workouts. Keep in mind that what you eat isn't immediately available anyway, so having an orange earlier in the day still provides the same benefit without having it by itself. Hope this helps!
(Good question Sara Ezell!)