Musnad Ahmad 33:552

Harun ibn Ma'ruf narrated to us, Ibn Wahb narrated to us, 'Amr ibn al-Harith narrated to me, Abu al-Zubayr narrated to me, that he heard Jabir ibn 'Abd Allah mentioning that the Messenger of Allah (peace and blessings of Allah be upon him) said: "In what the rivers and the clouds have watered, there is a tenth (tithe). And in what the water-drawing camel has watered, there is half a tenth."