Short answer
It depends on the amount of water. In the Hanafi school, a small amount of still water, such as a bucket, sink or bath, becomes najis from any impurity, even if nothing changes. Running water and a large body of water stay pure unless the colour, taste or smell of the impurity shows. The Shafi'i and Hanbali schools set the line at two qullas, and the Maliki school looks only at change.
Running water, and still water of a large body (traditionally ten by ten cubits with enough depth that scooping does not expose the bottom), does not become najis unless a quality of the najasah appears in it. Still water less than that becomes najis by any najasah that falls into it, whether or not it changes.
Other madhhabs
- Shafi'i
- Water of two qullas or more stays pure unless it changes; less than that becomes impure by any impurity, with small excused amounts.
- Maliki
- Water only becomes impure if the impurity changes its colour, taste or smell, whatever the amount; a little impurity in a little water makes it disliked to use.
- Hanbali
- Similar to the Shafi'i school, with two qullas as the dividing line.
Explanation
The Prophet ﷺ forbade urinating in still water and then bathing in it. The Hanafi school takes this to mean that a little impurity spoils still water that cannot carry it away. He also said that water of two qullas does not carry impurity, which the Shafi'i and Hanbali schools use as their measure, and that water is pure and nothing makes it impure, which the Maliki school stresses.
The Hanafi categories in practice:
- Small still water: a bucket, jug, bowl, sink basin, bath or small tank. If a drop of urine, blood, alcohol from wine, or a dead mouse falls in, the water is najis. Pour it away and wash the container.
- Large still water: a pond or tank of at least ten by ten cubits of surface, about 225 square feet (about 21 square metres) in the conversion common in South Asian fatwas, and deep enough that scooping water does not expose the bottom. It stays pure unless the impurity changes it.
- Running water: a river, a stream, tap water still flowing, or a tank with water running in and out at the same time. It stays pure unless the impurity shows in it.
- Swimming pools: most are large enough and are filtered, so they are treated as large or running water.
Not everything that falls in counts as najis. Flies, ants and other creatures without flowing blood do not spoil water: see dead insects in water. A cat drinking from it does not make it najis either: see cats. If you only suspect something fell in, the water stays pure: see doubts about najis.
Hanafi books give detailed rules for purifying a well by drawing out a set number of buckets. If this affects your home water supply, ask a local scholar. For which waters can be used at all, see water for wudu.
The evidence
Sahih al-Bukhari 239
Sahih... لا يبولن احدكم في الماء الدائم ...
"None of you should urinate in still water."
The Hanafi basis that small still water is spoiled by impurity.
Sunan Abi Dawud 63
Grading to be checked... اذا كان الماء قلتين لم يحمل الخبث ...
"When water reaches two qullas, it does not carry impurity."
The measure the Shafi'i and Hanbali schools use for large water.
Where the ruling is recorded
- al-Hidaya, Kitab al-Taharat, chapter on water. The difference between running water, large still water and small still water.
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Taharah, chapter on water. The ten by ten measure adopted by later Hanafi scholars, and purifying wells.
- al-Mughni (Ibn Qudama), Kitab al-Taharah, chapter on water. The two qullas and the views of the schools.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.