Musta’mal Water
The law of musta’mal water is thahur, holy and purifying like absolute water.
Musta’mal water is water that drips or flows from the body parts of people who perform ablution or take a big bath.
Rubayyi’ bint Mu’awwidz narrated that Rasulullah saw. wiped his head with ablution water remaining on hand.

Mus’tamal means something that is worn. Mus’tamal water is water that has been used for purification. Sayyid Sabiq in Fiqhus Sunnah says:
وهو المنفصل من أعضاء المتوضئ والمغتسل
“Musta’mal water is water that falls from the limbs of a person performing ablution or bathing” (Fiqhus Sunnah, 1/18).
Evidences about the purity of musta’mal water
Musta’mal water that has been used to remove hadats has a holy status and is not unclean, unless it is musta’mal water that has been used to remove impurity and changes one of its properties.
The following are among the arguments from Mausu’ah Fiqhiyyah Muyassarah Syaikh Husain Al ‘Awaisyah:

First:
Hadith from Al Miswar radhiallahu’anhu:
وإِذا توضَّأ النّبيّ – صَم قتتلون على وَضوئه
“When the Prophet sallallaahu ‘alaihi wa sallam performed ablution, they (the companions) almost killed each other (because they fought over) his ablution marks” (HR. Al Bukhari 189).
The companions performed tabarruk with the water used from the ablution of the Prophet sallallaahu’alaihi Wasallam. If the musta’mal water is unclean, then of course it will not be fought over by the friends and will be prohibited by the Prophet sallallaahu’alaihi Wasallam.
Second:
عَلَيْهِ وَسَلَّمَ – وهو يُقال له: إِنَّه يُستقى لك مِن بئر بُضاعة – وهي بئر يُلقى فيها لحوم الكلاب والمحايض وعُذَر النَّاس– فقال رسول ال له – صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ – : “إِنَّ الماء طهور، لا ينجِّسه شيء“
From Abu Sa’id Al Khudri radhiallahu’anhu, he said: I heard Rasulullah Shallallahu’alaihi Wasallam was once asked about the water taken from bidha’ah wells, namely wells where dog carcasses, sanitary cloths and dirt are usually thrown. So Rasulullah Sallallahu’alaihi Wasallam said: “water is holy, it cannot be defiled with anything” (HR. Tirmidhi no. 66, he said: “hasan”, authenticated by Al Albani in Sahih Sunan Tirmidhi).
If dead dogs, sanitary napkins and feces do not make the water completely unclean as long as there is no change in its color, taste or smell, then what is more, water that comes into contact with someone’s skin through washing and washing certainly does not make it unclean.
Third:
َلْتُ فأتيتُ الرحل فاغتسلتُ، ثمَّ جئت وهو قاعد، فقال: “أين كنتَ يا أب اهرّ؟ “. فقلتُ له ، فقال: “سبحان الله يا أبا هرّ! إِنَّ المؤمن لا ينجُس“.
From Abu Hurairah radhiallahu ‘anhu he said: The Prophet sallallaahu’alaihi Wasallam met me when I was in a state of junub. Then he held my hand and walked with me until we reached a place, we sat down. Then I slipped away, I went home and took a shower. Then I went to him, at that time he was still sitting. He asked, “where are you, Aba Hirr?” Then I explained my reasons. Immediately he said: “Subhaanallah! “O Aba Hirr, indeed fellow believers do not defile each other” (HR. Bukhari no. 285, Muslim no. 371).
