Musannaf Ibn Abi Shayba 35:484

Abu Khalid al-Ahmar narrated to us, from Yahya ibn Sa'id, from 'Amr ibn Yahya ibn 'Umarah, 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 (charity) on less than five awsuq."