Sayyiduna Ibn Umar (may Allah be pleased with them both) narrates from the Prophet (peace and blessings be upon him) that: "When the water amounts to two large jars (qullahs), nothing makes it impure."
(b) This narration is reported from Za'idah as mawquf (a Companion's own statement), and that is what is correct.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1245