Praise be to Allah.
Firstly: the signs of the end of a woman’s menses
A woman may know that her menses has ended by one of two signs:
1 – dryness, which is when the bleeding and yellowish and brownish discharges stop. This may be determined by inserting a piece of cotton and the like, and it comes out clean with no trace of what we have mentioned above.
2 – the white discharge, which is a liquid like plaster. Many women do not see this white discharge. If there is dryness, that is sufficient to deem the menses over, and there is no need to wait for the white discharge.
Al-Baaji said in al-Muntaqa Sharh al-Muwatta’ (1/119): What is usually the case is that the end of menses may be verified in two ways:
[The first is] the white discharge. ‘Ali ibn Ziyaad narrated from Maalik that it resembles semen. Ibn al-Qaasim narrated from Maalik that is resembles urine.
The second thing is dryness. That is when a woman inserts cotton or cloth into her vagina, and it comes out dry with no trace of blood on it.
Women vary in that regard. Some of them usually see the white discharge, and some usually see dryness. In the case of one who usually sees one of the two, when she sees it, it is deemed that her menses has ended. End quote.
Secondly: Making sure that menses has ended
If your menses usually lasts for seven or eight days, then at the end of the seventh day you should look to see whether your menses has ended or not. It is not valid to wait for twenty-four hours, and then do ghusl without inserting a piece of cotton and the like, and without emission of the white discharge. That is for two reasons:
The first is that menses may end at the end of the seventh day, so your method could result in missing prayers and obligatory fasts.
The second is that menses may continue until after the eighth day, in which case your ghusl – without checking whether menses has in fact ended – will not be valid.
Based on that, you must check, by inserting cotton, and it is not sufficient merely to wipe the outside, let alone waiting for some time and then doing ghusl.
The evidence for that includes the following:
The report narrated by Maalik in al-Muwatta’ (130) from Umm ‘Alqamah, that she said: The women used to send to ‘Aa’ishah the Mother of the Believers vessels in which were their rags, on which there was yellowish discharge from menstrual bleeding, asking her about prayer, and she would tell them: Do not hasten until you see the white discharge.
What is meant by that is the sign of menses having ended.
Al-Bukhaari also narrated it in a mu‘allaq report (The Book of Menses, Chapter on the Beginning and End of Menses).
The word translated here as vessel refers to a small vessel in which a woman would keep her perfume and other belongings. See an-Nihaayah by Ibn al-Atheer (2/246).
The word translated here as rags refers to pieces of cotton.
They did not only insert pieces of cotton; rather they would also send it to ‘Aa’ishah (may Allah be pleased with her), seeking reassurance that their menses had indeed ended.
Some of the scholars think that it is not necessary for a woman to check whether her menses has ended in the depths of the night. Rather she should check close to the time of prayer, and she should do that before she goes to sleep and at the time of Fajr prayer, i.e., before sunrise.
Al-Bukhaari (may Allah have mercy on him) said in his Saheeh: Chapter on the Beginning and End of Menses … The daughter of Zayd ibn Thaabit heard that some women were calling for lamps in the depths of the night, to see if their menses had ended.
She said: The women never used to do this, and she criticized them for that. End quote.
This report was narrated by Maalik in al-Muwatta’.
Ibn ‘Abd al-Barr (may Allah have mercy on him) said: The daughter of Zayd ibn Thaabit only criticized the women for checking themselves at a time other than the time of prayer or close to the time of prayer because the depths of the night is not a time for prayer; rather the women should check themselves for the purpose of prayer, then if they see that their menses has ended they should prepare themselves to do ghusl because they have to pray.
See also: al-Muntaqa Sharh al-Muwatta’ by al-Baaji (1/120); Fath al-Baari by Ibn Hajar (1/421).
Ad-Dardeer said in ash-Sharh al-Kabeer (1/172): The menstruating woman does not have to – and it is neither obligatory nor recommended – to check whether her menses has ended before Fajr, so that she can catch up with Maghrib and ‘Isha’ and fast. Rather that is makrooh (disliked) because that is not how it is done. And because the imam said: I do not like that. Rather she should check herself before going to sleep at night, so that she will know whether she can pray qiyaam and fast. The basic principle is that she is as she was, but at the time of Fajr and other prayers she must do that, but not immediately, until when there is only enough time left to do ghusl and pray, that becomes an urgent obligation. End quote.
And Allah knows best.