Is it permissible to place a marble slab over the grave of the deceased, to please his mother?

If the intention behind placing marble on a grave is to adorn it, then it is forbidden by Islamic law (haram), and it is not permissible to obey one's mother in this matter, because obedience is only in what is good and permissible. However, if the intention is to protect the grave or to mark its location, then there is no harm in it. It is advisable to counsel the mother by explaining that this is contrary to the Sunnah and a waste of money, and that what benefits the deceased are righteous deeds, supplications (du'a) and seeking forgiveness for them, and giving charity on their behalf.