Abu Dawud

274. Commentary:
The blood that comes to every adult woman as part of the monthly cycle is called menstruation (hayd), and it is a sign that her womb is empty. From the onset of puberty, every woman generally becomes familiar with her own cycle. Usually, this blood is darkish in color, but if there is a disorder in this system and the bleeding exceeds the usual pattern, it is called irregular bleeding (istihada), and its color is also somewhat different. The blood that comes after childbirth is called postnatal bleeding (nifas). The days of menstruation (hayd) and postnatal bleeding (nifas) are considered days of impurity, but the days of irregular bleeding (istihada) are counted as days of purity, on the basis that this is a condition of illness.

The ruling regarding istihada is as follows: if a woman knows the dates and duration of her menstruation and this condition (istihada) occurs, then she should leave the prayers during those specific days, and her husband should also remain apart from her. If there is variation in the dates and duration, then the days in which the blood is darkish should be counted as the days of menstruation. However, if the dates and duration are not fixed, and there is no distinction by color, or if the condition of istihada has existed from the very beginning, then six or seven days, or the number of days according to the habit of her female relatives, should be determined as the days of menstruation. During these days, she should abstain from prayer, fasting, and marital relations. After these days are completed, she should perform a ritual bath (ghusl), and then begin prayer and fasting, and thereafter perform ablution (wudu) for every prayer. If she has the strength to perform a ritual bath (ghusl) for every prayer, that is even better. The husband will also be permitted to have marital relations. The ahadith regarding istihada should be studied in light of this brief introduction.

209. Commentary:
➊ Our esteemed researcher has declared this narration weak in terms of its chain of transmission, but the narration is correct in meaning because it is supported by other hadiths. Furthermore, the researcher of the book himself has acknowledged the corroboration of some parts of the hadith, and the narration of Aisha radi Allahu anha also serves as supporting evidence for it. See: [صحيح مسلم، الحيض، حديث : 333]
For further details, see: [الموسوعة الحديثية مسند أحمد : 123/44]
➋ If a woman used to have regular menstruation (hayd) and later began to experience irregular bleeding (istihada), then she should count as menstruation only those days in which she previously used to menstruate, and refrain from prayer during those days. On the remaining days, even if bleeding continues, she should continue to perform prayers and other acts of worship. However, after the days of menstruation end, she should perform a ritual bath (ghusl); there is no need for further ritual baths. And if she has experienced irregular bleeding from the beginning, then she should distinguish between menstruation and istihada by observing the color (of the blood). But if she cannot distinguish even by color, then she should consider any six or seven days in the month as menstruation, or adopt the monthly habit of her close female relatives, then perform a ritual bath (ghusl) and begin to pray.
➌ The cloth should be tied so that drops of blood do not soil the clothes and body.

354. Commentary: See Sunan an-Nasa'i, Hadith: 209 and its benefits and issues.

359. Commentary:
Although this narration is weak in its chain of transmission, the same point has been mentioned in all other narrations as well, which are authentic.

Commentary:
(1)
This ruling applies in the case when the duration of the regular habit is known.

(2)
Just as in the case of istihada (non-menstrual vaginal bleeding), if someone has another illness in which their ablution (wudu) does not remain valid—for example, frequent passing of wind or the dripping of urine—then such a person is legally considered excused (ma‘dhur) according to the Shari‘ah. They should not abandon the prayer; rather, they should perform a new ablution (wudu) for each prayer, and with that ablution, they may perform all the obligatory, Sunnah, and supererogatory (nafl) prayers related to that prayer time.

(3)
This narration is also authentic due to its meaning being in agreement with other authentic narrations.