Musnad Ahmad 31:930

Waki’ narrated to us, from Sufyan, 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 (charity) on what is less than five wasqs of dates, nor on grain."