Sayyiduna Abu Sa'id (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "There is no zakah on less than five uqiyahs of silver, and there is no zakah on less than five camels." Sufyan (may Allah have mercy on him) said: An uqiyah is forty dirhams.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7513