Is ablution valid if there is dirt (Wasan) under the fingernails preventing water from reaching the skin?
Chapter on the Obligation of Purification
Al-Mughni
Book of Purification
Primary text
If there is dirt under the fingernails that prevents water from reaching what is underneath, Ibn Aqil states that the purification is invalid until this dirt is removed. This is because the area of the hand is covered by something not naturally belonging to its creation in a way that blocks water, while reaching the skin is possible and causes no harm, thus resembling a case where wax or something similar is present. The Prophet (peace be upon him) criticized them for having filth (Qalha) such that the smell of what was under their nails reached him, indicating that the issue was the offensive smell, not the invalidation of their purification. Moreover, such dirt is usually concealed, resembling what hair conceals on the face.
Supporting text
It is arguable that removing this dirt is not obligatory, because this is customary concealment. If its washing were obligatory, the Prophet (peace be upon him) would have clarified it, as delaying clarification at the time of need is impermissible.