Is it permissible to withhold a son's share of his deceased father's inheritance, specifically his share in the family home, for the benefit of his mother upon the father's death?

It is permissible to make a waqf (endowment) or a gift of a portion of one's wealth to one's mother out of honor and dutifulness to her. However, stipulating that this takes effect upon the death of the endower is impermissible, as it is considered a bequest to an heir. This is based on the Prophet's (peace be upon him) saying: "Indeed, Allah has given every rightful person his right, so there is no bequest for an heir," unless the heirs permit it.

This can be achieved by gifting your share of the inheritance to your mother, or by executing the waqf in a way that your mother benefits from its yield during your lifetime.

It should be noted that the "life tenancy condition" (such as saying: "I have endowed it to you for the duration of your life") is invalid according to the majority of jurists (Hanafis, Shafi'is, and Hanbalis). In such a case, it becomes the property of the life tenant and their heirs after them, and it never reverts to you.

It has been authentically reported from the Prophet (peace be upon him) that he made the life tenancy (al-umra) for the heir, and his saying: "Do not grant life tenancies (tumiru) nor life estates (turqibu), for whoever is granted something as a life tenancy or life estate, it becomes a path for inheritance."

In summary, it is permissible to gift to your mother or make a waqf for her from your wealth whatever you wish, but do not make it contingent upon your death, lest it become a bequest to an heir.