Purity: Ruling on Doubt Between Mixed and Unmixed Water

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 47

Water that was unmixed and it is not known whether that water has become mixed is deemed to be unmixed, meaning that it purifies an impure object, and wuḍūʾ and ghusl performed with it are valid (ṣaḥīḥ). Furthermore, water that was mixed and it is not known whether that water has become unmixed is deemed to be mixed, meaning that it does not purify an impure object, and wuḍūʾ and ghusl performed with it are invalid.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)