Definitions of leftover water:

What is left in a container after someone/something has drunk from it.

The fundamental rule concerning leftover water is that it’s pure until proven otherwise. The following are examples of leftover water:

Pure Leftover Water:

1- Leftover water from a human being

The human being is pure, whether a Muslim or disbeliever. If he or she is a Muslim, their state is referred to in the Prophet’s (peace be upon him) statement, “Indeed the believer does not become impure.” (Agreed upon)

And in the case of the disbeliever, the Prophet (peace be upon him) made wudu’ (ablution) with the left over water of a disbelieving woman. As to Allah’s saying, “The mushrikun (polytheists) are najas,” it refers to the inner impurity from their disbelief in the Oneness of Allah and in associating others with Him in worship.

It is reported that the Prophet (peace be upon him) used to drink from what `A’ishah (may Allah be pleased with her) had drunk while she was menstruating, and he used to put his mouth on the place where she drank from. (Muslim)

2- Leftover water from a cat

The Prophet (peace be upon him) said of the cat that drank from a container: “It (the cat) is not impure. It is merely one of your companions who frequently wanders amongst you.” (At-Tirmidhi)

3- Leftover water from animals whose consumption is permissible, as well as mules, donkeys, carnivores, hunting birds and their likes.

It is pure. This is because the basis of all things is purity when there is no evidence proving otherwise. It is well known that the Prophet (peace be upon him) used to ride donkeys as did others during his time. This proves that donkeys are not impure.

Impure Leftover Water:

1- Leftover water from a dog

The Prophet (peace be upon him) said: “The purification of any of your containers in which a dog has licked is by washing it seven times, the first (of them) with soil.” (Agreed upon)

2- Leftover water from a pig

The leftover water of a pig is filthy due to Allah’s (exalted be He) saying:

… And the meat of swine, for verily it is rijis (filth)… (Al-An`am 6:145)

Therefore that which comes from it would be filth also.

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Source: www.fiqhenglish.com.