Sayyiduna Abu Hurayrah (may Allah be pleased with him) reports this saying of the Prophet (peace and blessings be upon him): "Zakah is not due on less than five uqiyah (of silver), zakah is not due on fewer than five camels, and zakah is not due on less than five wasq (of grain)."
Ahmad recorded it, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الزكاة / 4379