The discussion of this issue is in two sections: first, concerning purity between two [instances of] blood; and second, concerning the ruling of the blood that returns thereafter.

As for the first, whenever a woman sees purity, she is pure; she performs ghusl, and prayer and fasting become incumbent upon her, whether she sees it during her habit or after it has elapsed. Our companions did not distinguish between a short period of purity and a long one, due to the saying of Ibn Abbas: "As for when she sees purity for an hour, let her perform ghusl." It is supported that the cessation of blood, whenever it is less than a day, is not purity, based on the narration we have cited regarding nifas (postnatal bleeding), that she does not pay heed to a purity of less than a day. This is the correct [opinion], if Allah wills, because blood flows at one time and stops at another. In mandating ghusl upon someone who is pure for an hour after an hour is hardship, which is negated by His saying, Glory be to Him: {And has not placed upon you in the religion any difficulty} (Quran 22:78). Furthermore, if we were to treat the cessation of blood for an hour as purity, and she does not pay heed to the blood that comes after it, it would lead to her not having an established menstruation. Therefore, according to this, the cessation of blood for less than a day is not purity, unless she sees what indicates it, such as if its cessation is at the end of her habit, or she sees the qassah al-bayda' (white discharge), which is something white that follows menstruation, called the tariyyah. This was narrated from our Imam. It was also narrated from him that the qassah al-bayda' is the cotton swab that the woman inserts; if it comes out white as it went in, with no change upon it, then that is the qassah al-bayda'. This was reported from al-Zuhri, and also narrated from our Imam. Abu Hanifah said: The cessation [of blood] between two [instances of] blood is not purity; rather, if she fasted during it as an obligatory fast, it would not be valid, she would be required to make it up, prayer would not be obligatory upon her during it, and her husband would not come to her, so the two [instances of] blood and what is between them would be menstruation. This is one of the two opinions of al-Shafi'i, because blood flows at times and stops at others, and because if it were not part of the menstruation, it would be counted as part of its duration. Our evidence is the saying of Allah, the Exalted: {And they ask you about menstruation, say: it is an impurity} (Quran 2:222). He described menstruation as being an impurity.