Musnad Ahmad 31:574

Ya'la narrated to us, Idris al-Awdi narrated to us, from 'Amr ibn Murrah, from Abu al-Bakhtari, from Abu Sa'id, who attributed it to the Prophet, may God bless him and grant him peace, who said: "There is no Zakat on what is less than five wasqs, and the wasq is sixty [sa's] sealed."