What is the ruling on the purity of water when used water falls into it, specifically regarding the quantity?

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The view held by the companions of Al-Shafi'i states that if the majority of the water is used water, it is forbidden (to use for purification); however, if the amount is minor, it is not forbidden. Ibn 'Aqil stipulated that if the amount falling into the water was such that it would prohibit the water if it were vinegar (a rapidly permeating substance), then it is prohibited; otherwise, it is permissible. The ruling, based on the actions of the Prophet (peace be upon him) and his Companions, should return to common usage (Urf). If the amount is excessive and egregious, it is prohibited; otherwise, it is not. If there is doubt regarding the quantity, the water remains in its state of original purity (*tahuriyyah*) because purity is the default condition, and doubt does not negate the original state.