Musnad Abu Ya'la al-Mawsili 42:227

Abu Khaythama narrated to us, Waki' narrated to us, Idris al-Awdi narrated to us, from 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "There is no Zakat (charity) on what is less than five wasqs."