From Sahih Bukhari (Hadith 2248):
Narrated Abu Al-Bakhtari:I asked Ibn `Umar about Salam (the fruits of) date-palms. He replied, "The Prophet (ﷺ) forbade the sale of dates till their benefit becomes evident and fit for eating and also the sale of silver (for gold) on credit." I asked Ibn `Abbas about Salam for dates and he replied, "The Prophet (ﷺ) forbade the sale of dates till they were fit for eating and could be estimated
Reference: Bukhari 2248