From Sahih Bukhari (Hadith 2246):
Narrated Abu Bakhtari at-Tai:I asked Ibn `Abbas about Salam for (the fruits of) date-palms. He replied "The Prophet (ﷺ) forbade the sale of dates on the trees till they became fit for eating and could be weighed." A man asked what to be weighed (as the dates were still on the trees). Another man sitting beside Ibn `Abbas replied, "Till they are cut and stored." Narrated Abu Al-Bakhtari: I heard Ibn `Abbas (saying) that the Prophet (ﷺ) forbade ... etc. as above
Reference: Bukhari 2246