Abu Sa'id Khudri (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: "There is no zakat on less than five awqiyah."
Hadith Referenceمسند الإمام الشافعي / كتاب الزكاة / 725
Hadith Takhrijاخرجه البخارى الزكاة، باب زكاة الورق (1447)۔