Praise be to Allah.
The four madhhabs are agreed that if what is emitted goes beyond its place of exit and spreads to a large extent, then cleaning oneself with pebbles (istijmaar) is not sufficient; rather it is essential to wash the area.
The reason for that is that istijmaar is a concession that was granted because of a widespread practice, so it should be limited to regular situations [when the impurity does not spread]. Anything beyond that [when the impurity spreads] remains subject to the original ruling, which is that the impurity must be removed by washing. End quote.
Al-Mawsoo‘ah al-Fiqhiyyah (4/121)
It is acceptable for a woman to clean herself with pebbles after defecating, according to scholarly consensus. This is clear.
But with regard to urination:
According to the Maalikis, cleaning with pebbles is not acceptable following urination in the case of a woman, whether she is a virgin or otherwise. They said: That is because the urine usually goes beyond the place of exit.
According to the Shaafa‘is, following urination in the case of a woman – if she is a virgin – whatever removes the impurity itself is acceptable, whether it is a piece of cloth or anything else. But in the case of a non-virgin, if the urine reaches the outside of the vagina, as is usually the case, then cleaning with pebbles is not sufficient, but otherwise it is sufficient.
It is mustahabb to wash the area in that case.
Among the Hanbalis, there are two views concerning the non-virgin:
1. that istijmaar is acceptable
2. that the area must be washed. End quote.
Al-Majmoo‘ah al-Fiqhiyyah (4.122)
See also: Mawaahib al-Jaleel by al-Hattaab (1/284); Minah al-Jaleel by ‘Ulaysh (1/105)
Ibn Qudaamah (may Allah have mercy on him) said:
A virgin woman is like a man, because the hymen prevents the spread of urine.
But in the case of the non-virgin, if urine is emitted in a stream and does not spread, then the same applies.
But if it spreads to the site of menstruation, in that case our companions said that it must be washed, because the site of menstruation and childbirth is different from the place where urine exits.
It may be that washing is not required, because this is usual in her case, therefore istijmaar is sufficient. Moreover, if washing were required, when this happens to many women, the Prophet (blessings and peace of Allah be upon him) would have explained that to his wives, because it is something that they would have needed to know.
End quote from al-Mughni (1/118)
The correct view is that with regard to this matter, a woman is like a man, whether she is a virgin or otherwise.
This is the view favoured by a number of well-versed scholars, such as al-Majd Ibn Taymiyah, the grandfather of Shaykh al-Islam Ibn Taymiyah, and al-Mardaawi. See: al-Insaaf (1/160)
Shaykh ‘Abdullah at-Tayyaar was asked:
It is said that it is stipulated that a woman should clean herself with water (istinja) after urinating, and this is said by those who claim to belong to the Maaliki madhhab. Is this view correct?
He replied:
I do not know of the basis for that. A woman is like a man: she may clean herself with water (instinja) or she may clean herself with pebbles (istijmaar), but if she follows the pebbles with water, that is better. End quote.
http://www.m-islam.com/new/s/1276
For more information, please see the answers to questions no. 9645 and 111813
And Allah knows best.
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