Musnad Ahmad 283:5

Yahya ibn Sa'id narrated to us, Shu'bah narrated to us, he said: Khubayb ibn 'Abd al-Rahman informed me, from 'Abd al-Rahman ibn Mas'ud ibn Niyar, he said: Sahl ibn Abi Hathmah came to us in our mosque and said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When you estimate (the yield of dates), take and leave; leave a third. If you do not find (it appropriate) to leave a third, then (leave) a fourth."