Q: Is this true that Allah does not order us not to leave any dry place in wudu’ and ghusl. If one, though one has washed a limb, does not know that a
part on the limb has been left unwashed, that limb is assumed to have been washed completely?
A: If you are sure that the place was left dry then it is compulsory for you to wash the dry area. One needs to wet himself according to his understanding and strength. Once you have accomplished the same the wudhu is done.
And Allah Ta’ala (الله تعالى) knows best.