Assalaamu alaykum. I read in your fatwa that full certainty regarding washing is not required in ablution and Ghushl (bath taken in state of major ritual impurity). My question is: suppose one washes his face three times and did his best to make sure that water reaches all parts and some part nevertheless remained unwashed without his knowledge, will his ablution be valid?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

There is no doubt that your question indicates that you suffer from some sort of Waswaas (obsessive whisperings). You should know that it is established that when a person washes his face thrice, applying water to the whole face, he has washed the whole face, leaving no part unwashed. Scholars stated that it is enough to believe that water has most likely reached the whole body in Ghusl; absolute certainty is not required in this matter. If you believe that what you are washing has most likely been washed fully, then this is sufficient and your ablution or Ghusl is valid whether or not the reality is what you believed.

Our advice to you is to ignore doubts and totally disregard such satanic whispers, as getting carried away with them leads to grave evil.

Allah knows best.