Musnad Ahmad 30:2098

Ali ibn Ishaq narrated to us, he said: Abdullah informed us, he said: Ma'mar informed us, he said: Suhayl ibn Abi Salih narrated to me, from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "There is no Zakat on what is less than five wasqs, and there is no Zakat on what is less than five awqiyahs, and there is no Zakat on what is less than five camels."