What is the ruling regarding water whose contamination source is unknown?
Chapter on Selling the Musarrah (Animal with milk retained in udder)
Al-Mughni
Book of Sales
Primary text
If water is found to be changed in condition, and it is unknown whether the change was due to impurity or something else, the water remains rulingly pure. This is because the default state is purity, and this state cannot be abandoned except through certainty or a strong presumption, neither of which has been established.
Supporting text
The basis for this ruling is the Hadith of Abdullah ibn Zayd, where the Prophet (peace be upon him) instructed a man who suspected passing gas during prayer not to stop until he heard a sound or detected an odor.