This is a slight urban myth. After all when you really think about it 3lb of muscle weighs exactly the same as 3lb of fat. The key difference is how much volume the fat takes up in comparison to the muscle. Muscle tissue is a lot denser than fat and so will take up less space in the body meaning that you will look slimmer. You might weigh the same but your body will be trimming down and you will be feeling healthier and most importantly you will look slimmer.
The how to get ripped arms lesson really is to stop obsessing over what the scales say and look at your body and how it feels. If you do enough regular exercise you will start to notice a huge difference in the appearance of your body.
Not only does muscle take up less space in comparison to fat but it is also much smoother in tone so will therefore give you a much more even look.