water becomes impure upon mere contact with impurity, even if none of its qualities is altered.
According to Abū Ḥanīfah, a “small amount” is any quantity such that when one side of it is stirred, the other side moves as well. For the Shāfiʿīs and Hanbalīs, “small” means whatever is less than two qullas.
By contrast, Imām Mālik, the Ẓāhirīs, Shaykh al-Islām Ibn Taymiyyah, Ibn al-Qayyim, Shaykh Muḥammad ibn ʿAbd al-Wahhāb, the Salafī scholars of Najd, and many other researchers hold that water is not rendered impure by mere contact with filth unless one of its three properties—taste, color, or smell—is altered.
Those who judge water to be defiled simply by contact cite the implied meaning of the ḥadith of Ibn ʿUmar on the two qullas: in their view its implication is that water below two qullas “carries filth.” In another wording, “When it reaches two qullas, nothing renders it impure”; therefore, they conclude, whatever is less than that amount becomes impure upon mere contact.
They also appeal to the ḥadith that commands pouring out a vessel from which a dog has drunk, a ruling that, they argue, does not depend on any perceptible change in the water.
The Ḥanafīs see no conflict between their view and the ḥadith of the two qullas, for when two qullas are poured into one place, moving one side of the water does not cause the other side to move.
As for the evidence marshalled by those who restrict impurity to cases of discernible change, it includes the very ḥadith of the two qullas. They understand its meaning to be that water which has reached two qullas is not defiled by mere contact because impurities dissolve and disappear in it. The implied meaning, they say, is not binding: a smaller amount may still remain pure if none of its properties is altered, and it may become impure if a change does occur.
They also cite the report about throwing a bucketful of water over the Bedouin’s urine in the mosque, along with other proofs.
Ibn al-Qayyim said: “What the foundational principles require is that water does not become impure so long as the impurity has not altered it…"