Musnad Ahmad 33:749

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Yahya ibn Habban narrated to me, from his paternal uncle Wasi' ibn Habban, from Jabir ibn Abdullah al-Ansari: "That the Messenger of Allah, peace and blessings of Allah be upon him, commanded that. From every ten loads (wasq) of harvested dates, one load."