Musnad Ahmad 31:415

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Yahya, from his father, from Abu Sa'id, from the Prophet, peace and blessings of Allah be upon him, that he said: "There is no Zakat on less than five camels, nor on five wasqs, nor on five awqiyah."