Muhammad bin Sa'd al-Jallab says: In Madinah I asked Isma'il bin Abi Uways about the sa' of the Prophet (peace and blessings be upon him), and he brought out an old sa' and said: This is the very sa' of the Prophet (peace and blessings be upon him). I measured it, and it was five and a third ratls.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7722