It is written in Durr-ul-mukhtar, and also in Radd-ul-mukhtar, which is an explanation of the former:

A minor ablution [an ablution for namaz] and a major ablution [ghusl] require using mutlaq water. In other words, mutlaq water is both clean and a cleaner. Mutlaq water is water that takes no other word besides its name and which is solely called water. Water from rain, brooks, streams, springs, wells, seas, and snow is mutlaq water. Mustamal water (explained before), najs water, flower essence, grape juice and the like, which are mentioned together with their kinds and properties, are not mutlaq water. These cannot be used for making an ablution or ghusl. They are called Muqayyad water. Zamzam water can be used for an ablution or a ghusl. It is not even makruh. It is permissible also to use water that has stayed for some time under the sun. But is tanzihi makruh.

Water issuing and dropping from trees, grass, fruit or from any climbing plant is clean. But an ablution or ghusl is not permissible with it or with any juice extracted from these plants.

When something clean is mixed with mutlaq water, if the amount of the substance mixed with the water is more than the water, the water becomes muqayyad. The substance mixed with water may become the greater part in four ways. Firstly, something solid, such as a sponge or grass, absorbs the water completely. Secondly, something which is not used in cleaning like soap is heated in the water. Meat broth and bean juice are of this sort. In this case the water becomes muqayyad water even if its three properties did not change and even if it did not lose its fluidity. Water heated with some cleaner such as soap becomes muqayyad if it loses its fluidity. Thirdly, a solid substance gets mixed with cold water. If the substance changes the name of the water after it is mixed with it, the water becomes muqayyad even if it is not viscous. An example of this is water containing saffron or sulfate of iron or water containing oak apple, if the amount of the substance dissolved in it is such that it can be used in dying or tannery. The same applies for the nabiz made from dates. Dates or dried grapes are put in cold water. When their sugar is transferred into the water, the water is then heated until it boils. After getting cold, it is filtered. This liquid is called nabiz. If it is filtered without being heated it becomes naqi'. When the name of the water does not change, if the water becomes viscous, or loses its fluidity, it becomes muqayyad. If it is still fluid, it remains pure, even if its three properties changed. Examples of these types are water dyed by saffron that has fallen into it and cold water whose color, odor or taste has been changed by beans, chick-peas, leaves, fruits or grass that has remained in it for some time. It is not permissible to make an ablution or ghusl with saturated solutions of salt.

The fourth possibility is when a liquid substance gets mixed with water. When a clean substance in liquid form flows into a small pool, if all three properties of the substance are unlike those of water and if two properties of the mixture have changed, it becomes muqayyad. If only one has changed, it does not become muqayyad. Water mixed with vinegar is an example of this type. When one or two of its properties are like those of the water, or if a property of the water which is unlike that of the substance has been changed in the mixture, it becomes muqayyad. Milky water is an example of this type because being odorless is common in both of them. And so is water mixed with melon juice because, being colorless and odorless is common in both of them. If all three of its properties are like those of water, or if the amount of the liquid mixed with water is more than the water or equal to the water, the mixture becomes muqayyad and it is not permissible to perform an ablution or ghusl with it. The case is the same when mustamal water [that which has been used in an ablution or ghusl] gets mixed with the water. This is so, if mustamal water is thought to be clean. The case is the same when mustamal water flows into a small pool or into a bath-tub or when a person without an ablution dips his hand or foot into it or he himself dives in it. So long as it is not known that the amount of water touching the skins of those who perform an ablution at a small pool is half of the total amount of the water in the pool, into which no water flows, or that any najasat, though little, has fallen into the pool, then it is permissible to perform an ablution at the pool. If a lot of people perform an ablution at a small pool whose water is changed daily and if their mustamal water falls back into the pool, it is permissible. But if very little najasat falls into the pool, it is not permissible to perform an ablution there. According to some scholars, if any limb is dipped and washed in a small pool, the whole pool becomes mustamal water. For this reason, at places rich with water, taking some water with the palms and washing the limbs outside the pool must be preferred to dipping the limbs into the pool. At places with scarce water, acting upon those scholars who say that it is permissible, an ablution and/or a ghusl can be made in the pool.

It is sahih, but it is haram, to make an ablution with usurped water.

It is permissible to make an ablution or ghusl with mutlaq water in which an animal without fluid blood has died. For example, it is permissible with water in which there is a dead bee, scorpion, bed-bug or mosquito. It is not permissible if there is a dead leech that has sucked blood. A silk worm and its eggs, those worms living in excrement, tapeworms and fruit worms are clean. But the najasat smeared around them is najs.

When an aquatic animal dies in some water, such as fish, crabs, frogs, it is permissible to make an ablution or ghusl with this water. Also, of toads and snakes living on land, when those without fluid blood die in the water, it is permissible. If all such animals die after being taken out of the water and their dead bodies fall into the water, it is still permissible. Also, if a frog gets broken to pieces in the water, it is still permissible. But the water cannot be drunk. For its flesh is haram (to eat). When an animal that is born on land and lives in water, such as a duck or a goose, dies in a small pool, the pool becomes najs.

When little najasat falls in a small pool, according to Hanafi Madhhab, or in any pool of water that is smaller than qullatayn, according to Shafi'i Madhhab, the water becomes najs even if its three properties did not change. Men cannot drink it, nor can it be used in cleaning. If its three properties change, it is like urine and cannot be used in anything. A qullatayn is five hundred ritls. One ritl is 30 dirhams, and one dirham 3.36 grams. Hence, one qullatayn is 220 kilograms.

Water whose three properties have changed as a result of staying too long does not become najs. If the reason for any smelling water is not known, it will be considered to be clean. There is no need to inquire, to ask others. In order to oppose the (group) of Mutazila, it is sometimes advisable to make an ablution at a pool which is besides a river.

If some najasat, whether it can be seen or not, falls in a river or a large pool, in Hanafi Madhhab, or in an amount of water equal to qullatayn, in Shafi'i Madhhab, or in any amount of water, in Maliki Madhhab, it is permissible to make an ablution at its any side where any of the three signs of the najasat, such as its color, odor or taste is not evident. For example, when there is a carrion, when a man or an animal urinates or when a beast of prey drinks water (from a stream), if there is no sign of them at its lower parts, it is permissible, (to make an ablution). According to some scholars, being permissible necessitates that the amount of water that has touched the najasat be less than the amount of water that has not touched it. The water does not have to flow continuously. When any water poured on some najs place flows for one meter and if its three properties are gone, it becomes clean. When clean water in one container and najs water in an other container are poured from a level one meter above the ground and get mixed with each other in the air, the water that falls on the ground will be clean.

Water that can sweep along a chaff of straw is called flowing water. A square pool with each side measured as ten zra' [about 4.8 meters] is called (large pool), which covers an area of a hundred square zra', that is twenty-three square meters. A circle whose circumference is 17 meters covers an area of 23 square meters. There is no harm in its being shallow. If a person digs a ditch from a hollow and makes an ablution as the water in the hollow flows through the ditch and the mustamal water (used by him) accumulates in another hollow from which someone else digs another ditch and makes an ablution with the flowing water and the mustamal water accumulates at some other place and from there another ditch is dug and so forth, the ablutions of all these people will be accepted. The flowing water is clean until any sign of najasat is noticed. In this example mustamal water is considered to be najs. A small pool or a bath basin into which water flows continuously and which overflows continuously, [or from which water is consumed continuously and the duration between two consumptions is not so long as to let motion of the water stop], is the same as flowing water. An ablution can be made at any side of such things. Mustamal water must flow over the top. If it flows through a hole at the bottom, it is not like flowing water. It is not a condition that the pool be so small that all the mustamal water can flow away. If the surface of the pool is frozen, if (it is seen that) the water does not touch the ice when the ice is pierced, it is the surface of the water in the pool. If the water touches the ice, the hole will be the surface of the water. If clean water flows into najs water from one side and makes it overflow from the opposite side, its sides where there are no signs of najasat left are clean. When so much water as it contained has overflowed, it becomes clean all over. The overflowing water is clean so long as the signs of najasat are not noticed. So are such containers as bowls and buckets. For instance, if a najs bucket is filled and overflows, when none of the three signs of najasat is