Musnad Ahmad 31:581

Ishaq ibn Yusuf and Abd al-Razzaq narrated to us, they said: Sufyan informed us, from Isma'il ibn Umayyah, from Muhammad ibn Yahya ibn Habban, from Yahya ibn 'Umarah, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There is no Zakat on less than five awsuq (loads) of grain or dates. There is no Zakat on less than five awaq (ounces) of silver. There is no Zakat on less than five camels."