Chapter 80 : Some of the Melachos (Work) that are Prohibited on Shabbos
A garment or anything similar that is soiled, may be wiped with a rag or something similar, but water may not be spilled on it because spilling water on it is equivalent to washing. Therefore, if a child urinates on a garment, it is forbidden to pour water on it. (But if he urinates on the ground or on a wooden vessel or hide, it is permitted to pour water on it.) When you wash your hands and want to dry them with a towel, you should rub your hands together well, to remove the water, so that only a little water remains on them; (for the little water that you dry is considered soiling and is not considered washing). When a colored towel is used there is no cause for concern in any case, (since such a towel is not likely to be washed in this manner).
If a barrel contains water or other liquids, it is forbidden to cover it with a cloth which is not set aside to cover it, lest it be wrung out. But with a cloth set aside for this purpose, it is permitted to cover it, for since it is set aside for that purpose, we have no reason to fear that it will be wrung.
If water spilled on the table or any similar surface, it is forbidden to wipe it with a garment which you value, for since it absorbs much water you might wring it out. Similarly, you may not use a towel to dry cups or other vessels having a narrow opening, because the liquid will be squeezed out of the towel.