Musnad Ahmad 25:1314

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from 'Amr ibn Murrah, from Abu al-Bakhtari al-Ta'i, who said: I asked Ibn 'Abbas about the sale of date palms. He said: "The Messenger of Allah (ﷺ) forbade the sale of date palms until one eats from them, or is eaten from them, and until they are weighed." I said: "What is weighed?" A man who was with him said: "Until they are estimated."