Summary of answer:
Praise be to Allah
If something impure falls into water, it will affect the water in one of the following three ways:
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)
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)
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)