Musnad Ahmad 27:871

'Abd al-Wahhab ibn 'Abd al-Majid narrated to us, from Ayyub, from Nafi', from Ibn 'Umar, that the Messenger of Allah (ﷺ) forbade al-muzabanah. Nafi' used to explain it: "The fruit is bought by its estimation (while still on the tree) for dates of a specified measure; if it increases, it is for me, and if it decreases, it is against me."