What is the ruling on used water mixing with clean water when performing ghusl?

The water used for an obligatory purification is pure, not impure, but it is not purifying. Therefore, according to the majority of scholars, it is not permissible to purify oneself with it after it has been used. If a small amount of used water falls into other water, it does not harm it, and it is permissible to purify oneself with it, as it is difficult to guard against this. However, if the used water is abundant and excessive, then it is impermissible to purify oneself with it according to one of two narrations. The followers of Shafi'i hold that it is impermissible if the majority is used water, but not if it is the minority.