Critics point to the hadith of the Well of Buda'ah - into which refuse would sometimes fall - and claim it shows the Prophet (PBUH) taught Muslims to perform wudu with filthy water. It teaches nothing of the sort. The hadith is a precise legal ruling about when water remains pure, not an endorsement of washing in sewage.
The hadith
Abu Sa'id al-Khudri reported that the Prophet (PBUH) was asked about the Well of Buda'ah, into which menstrual cloths, dead animals, and other refuse would sometimes end up. He replied:
Water is pure, and nothing makes it impure.
Sunan Abi Dawud 66
The point is a ruling about the nature of water - not permission to drink or wash with visibly filthy water.
Contact with impurity does not automatically defile a large body of water
The Well of Buda'ah held a substantial amount of water. Impurities occasionally reached it, but they did not necessarily change the water itself. Islam does not declare a whole body of water impure simply because some impurity has touched it. The Prophet (PBUH) made this explicit elsewhere:
If the water reaches two qullahs, it does not carry impurity.
Sunan Abi Dawud 63; Jami at-Tirmidhi 67
A qullah is a large vessel; "two qullahs" denotes a sizeable quantity of water. Below that amount, jurists are stricter; above it, mere contact with a small impurity does not render the water unusable.
The jurists' actual standard
Classical jurists therefore distinguish between water that has merely had contact with impurity and water whose properties have been corrupted. The governing test is simple: if an impurity changes the water's smell, colour, or taste, the water is impure and cannot be used for purification. If it does not - as with a large well whose water remains clear and clean - the water stays pure.
So there is no image here of Muslims deliberately using a well full of decomposing animals and human waste. The report describes contaminants occasionally reaching the well; it does not say the water had become foul. If it had changed in smell, colour, or taste, the same law would rule it impure.
Water is the Quran's own means of purification
...and He sent down upon you from the sky water by which to purify you.
Quran 8:11
Islam treats clean water as the primary instrument of cleanliness. The Buda'ah ruling protects that principle: it prevents people from wrongly declaring an entire well or river unusable over a trace of impurity, while the smell/colour/taste test still bars genuinely foul water.
Conclusion
The hadith of the Well of Buda'ah establishes a straightforward principle of purity law: mere contact with an impurity does not automatically make a large body of water impure - but water whose smell, colour, or taste has changed is impure and may not be used. Far from telling Muslims to make wudu in filth, the hadith reflects the precision and reasonableness of Islamic purity law.
For the wider rulings on purification and impurities, see the related topics below.