Musannaf Ibn Abi Shayba 35:154

Ghundar narrated to us, from Shu'bah, from 'Amr ibn Murrah, from Abu al-Bakhtari, who said: I asked Ibn 'Abbas about the sale of date palms. He said: The Prophet, may Allah bless him and grant him peace, forbade the sale of date palms until one eats from them, or they are eaten from, and until they are weighed. I said: And what is meant by "weighed"? A man who was with him said: Until they are harvested (or stored).