Does a small amount of water become najis (impure) if some impurity falls into it but its characteristics do not change?

Praise
be to Allah

Firstly:

If something impure falls into water, it will affect the
water in one of the following three ways:

1.

The impurity changes one of the characteristics of the water
(colour, taste or smell). This water becomes impure according to scholarly
consensus, regardless of whether the water was of a great or small amount.

Ibn al-Mundhir said:

The scholars are unanimously agreed that if something impure
falls into water, of a small or great amount, and the impurity changes the
water in taste, colour or smell, then it is najis (impure) so long as it
remains like that, and it is not acceptable for the purpose of doing  wudoo’
or ghusl.

End quote from al-Awsat (1/260)

2.

If something impure falls into a large amount of water, but
it does not change any of its characteristics, be it taste, colour or smell.
This water is taahir (pure), according to scholarly consensus.

Ibn al-Mundhir said:

The scholars are unanimously agreed that if something impure
falls into a large volume of water, such as the Nile, or the sea and the
like, but it does not change it in colour, taste or smell, then it remains
as it was, and may be used for the purpose of purification.

End quote from al-Ijmaa‘ (p. 35)

3.

If something impure falls into a small amount of water, but
it does not change any of its characteristics, such as a drop of blood or a
drop of urine that falls into a vessel in which there is water, but it does
not change any of its characteristics – is it to be deemed impure because of
the impurity falling into it, or is it to be deemed pure because there is no
change in its characteristics?

The correct scholarly view is that the water is not to be
deemed impure unless it is changed by that impurity, whether the water was a
small or great amount.

This is the view of the Maalikis, and it is one report that
was narrated from Imam Ahmad. It was regarded as more likely to be correct
by Shaykh al-Islam Ibn Taymiyah and his student Ibn al-Qayyim, and was the
view favoured by many contemporary scholars such as Shaykh Ibn Baaz, Ibn
‘Uthaymeen, and the scholars of the Standing Committee for Issuing Fatwas
(may Allah have mercy on them).

See: adh-Dhakheerah by al-Quraafi (1/172); al-Mughni
by Ibn Qudaamah (1/39); Majmoo‘ al-Fataawa by Shaykh al-Islam
(21/32); ash-Sharh al-Mumti‘ (1/41).

They quoted as evidence for that the report narrated from Abu
Sa‘eed al-Khudri (may Allah be pleased with him) who said: It was said: O
Messenger of Allah, can I do wudoo’ from the well of Budaa‘ah? It is a well
into which are thrown menstrual rags, the flesh of dogs and other putrid
things. The Messenger of Allah (blessings and peace of Allah be upon him)
said: “Verily water is a means of purification and nothing makes it impure.”
Narrated by Abu Dawood (66), at-Tirmidhi (66) and an-Nasaa’i (326); classed
as saheeh by Imam Ahmad, Yahya ibn Na‘eem, at-Tirmidhi, an-Nawawi, Ibn
Mulaqqin and al-Haafiz Ibn Hajar. See: al-Majmoo‘ (1/82) and al-Badr
al-Muneer (1/381).

In this hadith we see that water is a means of purification
and nothing makes it impure. The scholars are unanimously agreed that it
becomes impure if it is changed by something impure falling into it, but
otherwise it remains as it originally was, which is pure.

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him)
said: This wording is general in meaning and applies whether the amount of
water is small or great, and it is general in meaning and applies to all
kinds of impure things.

But if it is changed by that impurity, then it is only haraam
to use it because the impure substance is still there, so when using the
water it is like using the impure substance too. This is unlike the
situation when the impure substance has been completely absorbed and
disappeared, for water is a means of purification and there is no impurity
remaining in that case.

End quote from Majmoo‘ al-Fataawa (21/33).

Shaykh Ibn Baaz said:

The correct view is that anything less than two qullahs [a
qullah is a type of vessel] does not become impure unless it is changed (by
the impure substance), as is the case when the amount of water is two
qullahs or more, because the Prophet (blessings and peace of Allah be upon
him) said: “Verily water is a means of purification and nothing makes it
impure.” The Prophet (blessings and peace of Allah be upon him) only
mentioned two qullahs in particular in order to highlight the fact that
anything less than that needs to be checked and examined with care, not that
it becomes impure in all cases, because of the hadith of Abu Sa‘eed
mentioned above.

What we learn from that is that a very small amount of water
is usually affected by impurity, so it should be poured away, disposed of
and avoided

End quote from Majmoo‘ Fataawa Ibn Baaz (10/16)

The scholars of the Standing Committee for Issuing Fatwas
said:

The basic principle concerning water is that it is pure, but
if its colour, taste or smell is changed by contact with something impure,
then it becomes impure, whether it is a small or great amount. But if the
impurity does not change it, then it is valid as a means of purification.

End quote from Fataawa al-Lajnah ad-Daa’imah (5/84)

Whatever the case:

The dispute concerning this matter is an old one, and it is a
matter that is very complex, and there is a great deal of argument about it;
each party has its evidence on which it based its view.

Ibn al-Qayyim said concerning this matter:

On this issue there have been real battles and a great deal
of argument, with many different points of view – namely the issue of water
when it comes into contact with an impure substance which is absorbed
completely by the water and no trace of it is left at all.

End quote from Badaa’i‘ al-Fawaa’id (3/257)

Ash-Shawkaani said:

The general meaning of the hadith “nothing makes it impure”
indicates that the water does not become impure by mere contact with an
impure substance, and the hadith about the two qullahs may imply that it
does become impure by mere contact with an impure substance. So some
scholars said that the hadith of the two qullahs may be understood as
meaning that water is regarded as still being pure if there is contact with
an impure substance, unless it is less than two qullahs. Others said that
even if it is two qullahs or more, the water is deemed to become impure if
there are traces of the impure substance in the water. 

The former view is supported by the evidence that is quoted
by those who say that a small amount of water is rendered impure as soon as
contact with an impure substance occurs, even if no change occurs in the
water. 

This issue is very confusing and no one could reach the
correct view except a few. 

End quote from Nayl al-Awtaar (1/46) 

It is better for a person to err on the side of caution and
keep away from small amounts of water if he knows that some impure substance
has fallen into the water, even if none of its characteristics were changed
as a result, so as to avoid any grey area, especially if the amount of water
is very small. In that case it becomes more important to be more cautious
and refrain from using it (for purification). 

And Allah knows best.