Musnad Ahmad 31:263

Hammad ibn Khalid narrated to us, Abdullah—meaning al-Umari—narrated to us, from al-Ala’ ibn Abd al-Rahman, from his father, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Zakat on less than five camels, and there is no Zakat on less than five awaq (of silver), and there is no Zakat on less than five awsuq (of crops)."