Answered by Ustadha Sulma Badrudduja
Question:  When washing the limbs under a tap, is it better to let the water run over the area before rubbing the limbs or should I rub my limbs at the same time?  When washing my feet, often the soles of my feet are already wet to begin with and when washing then I have doubts whether the wetness is from the washing or from the floor. How should I overcome this?  I learnt that water becomes used upon parting from the limb being washed after the first washing. So if I to wash my face only once and the water drips on my foot while I am washing it, should I run fresh water over the area? Or can I ignore it and rub the “mixed” water on my foot all over my foot?
Answer: Wa alaikum assalam wa rahmatullahi wa barakatuh,
1. The point is to ensure that water covers the entire area. This is best done by moving the water along in a simple, gentle, and swift fashion on the arm with one’s other hand.
2. Take the means to ensure that you are washing your entire foot — the sole, the top, between the toes, etc. Generally speaking, the best practice is to cover the entire limb with each of the three washings. It is practically impossible that you would miss a spot after keeping your limb under running water and washing it with your hand completely three times. After doing this, ignore any doubts you have that you missed an area or that its dampness is from before.
3. Water that is used for wudu or ghusl (in order to lift ritual impurity or not) becomes “used water” once it leaves the limb. Used water is pure, but not purifying. Therefore, you cannot use it to purify yourself as in wudu or ghusl; but if you come in contact with it, it won’t affect your state of purity/impurity. If some drops onto your foot while it is under running water, you may ignore it because the tap water is continuously running over the area and purifying it. Also, a small amount of used water does not affect a large amount of purifying water.
One should be thorough in making wudu, but keep in mind that it is something that Allah has made simple. May Allah increase you in your diligence, practice, and seeking of knowledge.
[ref: Maraqi al-Falah]
And Allah knows best.
Checked & Approved by Faraz Rabbani