Musnad Ahmad 31:585

'Abd al-Rahman narrated to us, from Malik, from Muhammad ibn 'Abd Allah—meaning Ibn Abi Sa'sa'ah—from his father, from Abu Sa'id al-Khudri, from the Prophet (peace and blessings of Allah be upon him), who said: "There is no Zakat on what is less than five wasqs, nor is there Zakat on less than five awaq, nor is there Zakat on less than five camels."