Water used in wuduu or ghusl
Water which has been used in raf' al-hadath during wuduu' or ghusl is Taahir [pure] in itself and muTahhir for others [i.e. meaning if the water was collected as it dripped of the body parts of the person who made wuduu' or ghusl with it, it could be used again for wuduu' or ghusl].
This is the view of the Maaliki Madhab. However, they say that if you use such water again for wuduu' or ghusl you will attain purification but with karaahah [being disliked].
That the water is purifying [muTahhir] is also the view of the Zaahiriyyah and of a group among the Salaf. It is the chosen view of Ibn al-Mundhir, Ibn Taymiyyah, Ibn Baaz, al-'albaani, and Ibn Uthaymiin.
This view has also been transmitted as one saying from Abu Hanifah, al-Shaaf'i, and as one narration from Ahmad.
Evidence for this view
First, from the Book of Allaah:
وَأَنْزَلْنَا مِنَ السَّماءِ مَاءً طَهُورًا
وَيُنَزِّلُ عَلَيْكُمْ مِنَ السَّماءِ ماءً لِيُطَهِّرَكُمْ بِهِ
Understanding the evidence
The word Tahuuraa has been used for water. This means it can be used again and again to attain purity.
It is based on the paradigm of Fa'uul which signifies that an action is done again and again such as the words:
shakuur which means to be thankful again and again
Sabuur which means to be patient over and over again
Evidence from the Sunnah
إنَّ الماءَ طَهورٌ لا يُنجِّسُه شيءٌ
Understanding the evidence
The Hadiith is general. Water is purifying and nothing makes it impure. There is no exception except when the water's color, taste, or smell change.
Another evidence from the Sunnah is
إنَّ الماءَ لا يَجنُبُ
Understanding the evidence
This means that the ruling of Hadath does not transfer over to water.
Additionally, it is pure water which comes in contact with a pure body so it cannot lose its ability to purify.
Finally, it is used water and so attaining purity with it is allowed just as using water used to renew wuduu can be used to attain purity. There is no difference between the two.
Reference(s):
Mudawwanah al-fiQh al-Maaliki wa 'adillah
Volume 1 Page(s): 39-40