وَأَنزَلْنَا مِنَ السَّمَاءِ مَاءً طَهُورً‌ا (And We have sent down purifying water from the heavens - 25:48). In Arabic طَهُور (Tahur) means something which is pure by itself and which also purifies other things. Allah Ta’ ala has given this quality to water that it is clean by itself and possesses the property to cleanse other things also. The water we use comes down from the clouds in the form of rain, hail or snow. Then some of it flows on the surface of the earth and the rest seeps down into the ground. Under the ground there is a natural drainage system which takes the underground water to every nook of the earth and then it reappears on the surface in the form of springs, fountains etc. Sometimes the underground water is drawn out by digging wells to reach it. All these waters i.e. that which falls in the form of rain, or that which flows on the surface of earth in the form of rivers, streams or canals, or that which is preserved underground are pure in Shari` ah and have inherent property of cleaning other things. There is consensus of opinion in ummah on this matter and it is the express purport of the Holy Qur'an and Sunnah.

When the water is in large quantity as in the case of a pond, pool or a canal, it remains pure even if something filthy has fallen in it. On this point also there is a general consensus, unless the effect of filth is perceptible and the colour, taste and smell of water changes. However, if the quantity of water is small and some filth drops in it, then there is a difference of opinion among the jurists (مُجتھدین) on the nature and quantity of filth as well as the quantity of water in which the filth falls. Details on this issue are described by Mazhari and Qurtubi in their commentaries and are also available in the books on Fiqh.