Musnad Ahmad 31:929

Waki' narrated to us, Idris bin Yazid al-Awdi narrated to us, from Amr bin Murrah, from Abu al-Bakhtari, 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 what is less than five wasqs."