Well, there are different kinds of sauces but I guess you're asking about a tomato based sauce. So, here it goes.
First flush the stain from the back side with cool water. This will loosen it from the fabric. Next pretreat it with some laundry detergent. Just pour a small amount right onto the stain and leave it for several minutes. Rinse again with water and sponge afterwards with white vinegar. If the stain is still quite visible pour some prewash stain remover on it. Put the clothing into the washing machine.
If the stain remains after laundering do not let the clothing dry. Drying will set the stain permanently. Rub some liquid laundry detergent into the stain and leave it in warm water to soak for 30 minutes. Launder it again and put in some bleaching agent if possible and necessary (you can rub in some lemon juice if you want a mild bleaching agent).