Why was a distinction made between considering dryness from menstruation as purity, with the obligation to wait a day and a half if it occurs during the menstrual period, and specifying the duration of connection between a yellowish discharge (kudrah) and blood, such that if it exceeds two hours it is considered a lack of connection, even though yellowish discharge also has an ebb and flow?

A woman knows she is purified from menstruation by two signs: dryness or the white discharge (al-Qassah al-Bayda’). Dryness means inserting a piece of cotton and it comes out clean, free of blood, yellowish discharge, or brownish discharge. Scholars have differed regarding dryness: is it a sign of purity as soon as it is seen, even if for only an hour, or must one wait a day or half a day?

The first opinion: Waiting is necessary because temporary cessation of blood is common during menstruation. Ibn Qudamah said: “If the cessation of blood is less than a day, it is not considered purity unless there is an indication, such as its cessation at the end of her usual period, or seeing the white discharge.”

The second opinion: Purity occurs with dryness, even if for only an hour, whether it is at the end of the usual period or during it. This is the Hanbali madhhab (definition), and Ibn Qudamah preferred it. He stated in "Matalib Uli al-Nuha": "And whoever's blood ceases during her usual period, even for the shortest duration, she is pure and should perform ghusl (definition)."

The second opinion is more apparent, because the purification of the site from blood, yellowish discharge, and brownish discharge is a sign of purity. Thus, a completely clean site indicates the end of menstruation, and this is known by inserting cotton and it coming out pure white.