(a) It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) was asked: O Messenger of Allah! May we perform ablution with the water of the well of Buda'ah, when filth, rotting flesh, menstrual cloths and dead dogs are thrown into it? He (peace and blessings be upon him) said: "Water is pure; nothing makes it impure." [Sahih]
(b) Shaykh Ahmad (may Allah have mercy on him) said: The hadith is evidence for purifying oneself from a well into which no impurity has been thrown. If impurity is thrown into it, then the meaning of the hadith is: when the water amounts to two qullahs (large jars) and it (its taste, colour and smell) has not changed. This will be discussed in its proper place, if Allah wills.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 6