Abu Dawud al-Sijistani (may Allah have mercy on him) says: I heard Ahmad bin Hanbal (may Allah have mercy on him) say: "The faraq is sixteen ratls," and I heard him say: "The sa' of Ibn Abi Dhi'b is five and a third ratls." He was asked: What about the one who said eight ratls? He said: That is not preserved.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7719