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
Yahya bin Ya'mar reported that the Prophet (peace and blessings be upon him) said: "When the water amounts to two large jars, it does not carry filth, and there is no harm in it." The narrator says: I asked Yahya bin Aqil: The jars of Hajar? He said: Yes, the jars of Hajar. The narrator says: I think that each jar holds two faraq of water. Ahmad bin Ali added in his narration that a faraq is sixteen ratls.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1251
Ibn Jurayj says: Muhammad informed me: I asked Yahya bin Aqil: Which jars? He said: The jars of Hajar. Muhammad, the narrator, says: I have seen the jars of Hajar; in my estimation each jar was equal to two waterskins.
The first chain is better preserved, and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1252
Mujahid (may Allah have mercy on him) says: "When the water amounts to two large jars, nothing makes it impure." The narrator says: I said: What are "qullatayn"? He said: "Two large jars."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1253
Abd al-Rahim bin Sulayman (may Allah have mercy on him) says: We asked Muhammad bin Ishaq bin Yasar (may Allah have mercy on him) about "qullatayn", and he said: These are the jars in which water is drawn, and the large drinking vessels (dawariq).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1255
Sayyiduna Malik bin Sa'sa'ah (may Allah be pleased with him) narrated the hadith of the Mi'raj (Ascension) of the Prophet (peace and blessings be upon him); in it is that he (peace and blessings be upon him) was raised to Sidrat al-Muntaha (the Lote Tree of the Utmost Boundary). He (peace and blessings be upon him) said: "Its leaves were like the ears of elephants, and its fruits were like the jars of Hajar."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1259