The evidence for not giving zakah to ascendants and descendants
Question

Is it permissible to give zakah to my parents? Please note that I am a woman; is it obligatory for me to spend on them? What is the scholarly evidence for not giving zakah to ascendants and descendants?
Praise be to Allah.Firstly:
In the answers to questions no. 111811 and 111892, we stated that it is obligatory to spend on ascendants and descendants. Ascendants are the father and mother, and grandfathers and grandmothers on both the father’s and mother’s side. Descendants are children and grandchildren, both males and females.
As it is proven that it is obligatory to spend on them, it is not permissible to give zakah to them because if they are poor and the individual – whether man or woman – is well off, he or she is obliged to spend on them. If he or she gives them zakah in that case, then he (or she) has protected his own wealth and saved it, so it is as if he gave zakah to himself.
The basic principle of the scholars concerning that is: Everyone on whom an individual is obliged to spend, it is not permissible for him to give the zakah of his wealth to them.
Ibn Qudaamah said in al-Mughni (2/269): zakah should not be given to parents or children. Ibn al-Mundhir said: The scholars are unanimously agreed that it is not permissible to give zakah to one’s parents when the giver is obliged to spend on them, because giving zakah to them would mean that they no longer need him to spend on them, and spending on them would be waived, so the benefit of that would come back to him, and it would be as if he had given it to himself.
Similarly, he cannot give it to his child. Imam Ahmad said: zakah should not be given to one’s parents or to one’s child or child’s child, or to one’s grandfather or grandmother or daughter’s child. End quote.
An exception is made to that in two cases, according to some scholars:
The first case is when the ascendant or descendant is in debt; in that case it is permissible to give him zakah, because the father is not obliged to pay off his son’s debts, and the son is not obliged to pay off his father’s debts.
The second case is when the wealth of the one who is giving zakah is not sufficient to spend on his ascendant or descendant; in that case, he is not obliged to spend on them and he may give them zakah.
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said, as noted in al-Ikhtiyaaraat (p. 104): It is permissible to give zakah to parents and grandparents, and to children and grandchildren, if they are poor and he is not able to spend on them. The same applies if they are in debt, or are slaves who have contracts of manumission, or are wayfarers. If the mother is poor and has small [minor] children who have wealth, but spending on her from it will cause them harm, then she may be given zakah from their wealth. End quote.