Bukhari

Explanation:
When a woman performs the ritual bath (ghusl) after menstruation (hayd), she must use some fragrance at the private area to remove any odor. This has been emphasized to such an extent that even a woman in mourning (i.e., observing the waiting period after her husband's death) is permitted to do so, provided she is not in the state of ihram. "Kust" or "azfar" refers to costus wood (oud). Some have interpreted "azfar" as the name of a city in Yemen from where Indian oud used to be brought to the Arab lands. The narration of Hisham has also been transmitted by Imam Bukhari rahimahullah himself in the Book of Divorce (Kitab al-Talaq).

Hadith Commentary:
➊
Shah Waliullah Muhaddith Dehlawi rahimahullah writes in the explanation of the chapter headings that Imam Bukhari rahimahullah has established in this title that a menstruating woman may pray in the same clothes in which she spent her days of menstruation.
And the need for this issue was felt because, before Islam, women used to consider it necessary to change the clothes they wore during menstruation at the time of purification (tuhr).
But the Shariah clarified that if the garment worn during menstruation is not soiled with blood, there is no need to wash it; rather, prayer can be performed in it.
If there is any blood on it, then only the soiled part should be washed, and then prayer can be performed in it.
They used to rub it with their nails and then wash it with water.
Relying on the apparent meaning and for the sake of brevity, washing is not mentioned in the hadith.
The clarification regarding washing the garment has already passed in hadith (308), that when any of us would menstruate, at the time of purification she would scrape off the blood from her garment and wash it, then pour water over the remaining part of the garment and pray in it.

➋
According to some scholars, removal of impurity (najasah) can be done with any liquid besides water.
They have presented this hadith as evidence, because it explicitly mentions removal of impurity without water. However, this reasoning is not correct, because other ahadith clarify that merely rubbing with the nail was not sufficient, rather it would be washed, and then prayer would be performed in it. The use of saliva was because saliva contains saltiness, due to which the blood stain could be removed more easily.
Thus, the use of saliva was not for the complete removal of impurity, but rather to facilitate the removal of impurity.

➌
Other ahadith indicate that the Mothers of the Believers (azwaj mutahharat radi Allahu anhunna ajma‘in) had separate garments for menstruation and for the state of purity.
As is narrated from Umm Salamah radi Allahu anha.
Whereas in this narration it is stated that we, the Mothers of the Believers radi Allahu anhunna ajma‘in, had only one garment for both menstruation and purity.
Apparently, there is a contradiction between these narrations.
They can be reconciled in this way: the statement of Aisha radi Allahu anha relates to the early period of Islam, when there was hardship, and later, when the era of conquests came and Allah Ta‘ala granted abundance, separate garments for menstruation and purity were adopted. Also, the statement of Umm Salamah radi Allahu anha is not explicit regarding a separate garment during menstruation, because it is possible that by “menstruation garment” she meant the coarse cloth, etc., which women used as a precaution during menstruation, not a permanent garment.
Similarly, the statement of Aisha radi Allahu anha is not explicit that the same garment was used throughout the days of purity and menstruation.
Her statement could also mean that, apart from ordinary clothing, we did not have any specific type of cloth which was needed to stop the obligatory blood.
In short, there is no contradiction between the statements of Umm Salamah radi Allahu anha and Aisha radi Allahu anha; rather, both are correct in their respective contexts.

➍
From this, it is also understood that the sweat of a menstruating woman is pure, because if a woman’s sweat during menstruation were impure, then after the end of menstruation, her garment would be considered impure in any case, and it would be necessary to wash it, even if there was no blood stain on it. Whereas the hadith states that if there is no blood stain on the garment, there is no need to wash it.
Therefore, it is understood that, like a person in a state of major ritual impurity (junub), the sweat of a menstruating woman is also pure.
It is possible that this was the very purpose of Imam Bukhari rahimahullah in this chapter heading.