Musnad Ahmad 152:4

'Affan narrated to us, Shu'bah narrated to us, he said: Khubayb ibn 'Abd al-Rahman al-Ansari informed me, he said: I heard 'Abd al-Rahman ibn Mas'ud ibn Niyar, he said: Sahl ibn Abi Hathmah came to our gathering, and he narrated that the Messenger of Allah (may Allah bless him and grant him peace) said: "When you estimate (the yield of dates), then harvest, and leave a third. If you do not leave a third, then leave a fourth."