Is used water (*ma' musta'mal*) considered impure if it falls into pure water?

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If a small amount of used water falls into pure water, it is overlooked (pardoned). This is supported by the practice of the Prophet (peace be upon him) and his Companions, who performed ablution from bowls and pitchers, and performed Ghusl from large basins, which inevitably resulted in splashing back into the source water. For instance, the Prophet (peace be upon him) performed Ghusl with Maymunah from a basin containing traces of dough, and he and 'Aishah performed Ghusl from one vessel, both saying to the other, "Leave some for me." In such instances, it is impossible to avoid splashing. If the amount of used water that falls is great and excessive, it invalidates the purity of the water based on one narration.