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