From Sahih Bukhari (Hadith 2249):
Narrated Abu Al-Bakhtari:I asked Ibn `Umar about Salam for dates. Ibn `Umar replied, "The Prophet (ﷺ) forbade the sale (of the fruits) of date-palms until they were fit for eating and also forbade the sale of silver for gold on credit." I also asked Ibn `Abbas about it. Ibn `Abbas replied, "The Prophet (ﷺ) forbade the sale of dates till they were fit for eating, and could be weighed." I asked him, "What is to be weighed (as the dates are on the trees)?" A man sitting by Ibn `Abbas said, "It means till they are cut and stored
Reference: Bukhari 2249