Ibn Jurayj narrates through a chain, the mention of which I do not recall, that the Messenger of Allah (peace and blessings be upon him) said: "When the water amounts to two large jars, it does not carry filth (that is, it does not become impure)."
In the hadith he said: "by the jars of Hajar." Ibn Jurayj said: I have seen the jars of Hajar; a jar holds two waterskins, or two waterskins and something more.
Al-Shafi'i said: Muslim (bin Khalid) held that this "something more" was less than half a waterskin, or half a waterskin, so he would say: Five waterskins is the most that two jars hold, and two jars may be less than five waterskins. Al-Shafi'i said: Precaution is that a jar be reckoned as two and a half waterskins; so when the water amounts to five waterskins it does not carry impurity, whether it is in a jar or elsewhere, unless its smell, taste or colour appears in the water. He said: The waterskins of the Hijaz are large, so the water that does not carry impurity is reckoned only by large waterskins.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1250