All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaahs and that Muhammad is His slave and Messenger.
If an impurity contaminates the person’s body or clothes and it is not actual impurity, i.e. it has no mass, then it is not necessary to rub it during the purification process; rather, it is enough to pour water over it once and have the water run over it. It is not obligatory to rub the impurity unless rubbing is needed for removing it. The Shaafiʻi scholar Al-Khateeb Ash-Shirbeeni wrote:
"Whatever is contaminated with an impurity other than (saliva of a) dog and the like and (urine, stool, or vomit of) an unweaned baby boy fed only with breast milk, and this impurity is not actual, i.e. its existence is known with certainty yet it has no taste, color or smell, then it is enough to pour water over the contaminated spot so that water would flow beyond sprinkling. However, if the impurity is actual, then it is obligatory after removing its mass to remove its taste even if done with difficulty. If it is difficult to remove its color (as is the case with blood) or smell (as is the case with wine) due to the hardship, then there is no harm if they remain. However, if it is easy to remove the color or smell of the impurity, then there is harm if they remain (i.e. making water flow beyond sprinkling is not adequate). If both the color and smell of the impurity remain in the item, then the item is not duly purified given that their presence together strongly indicates that the impurity itself has not been removed in the first place." [End of quote]
Allaah knows best.