Purity: Ruling on Doubt Concerning Water’s Purity or Impurity

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 52

Water that was pure, and it is not known whether it has become impure or not, is pure. Water that was impure, and it is not known whether it has become pure or not, is impure.

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