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