Musnad Ahmad 31:754

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Yahya, 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 less than five camels, nor is there Zakat on five wasqs, or five awaq."