Sahih Bukhari Volume 3, Book 35, Hadith Number 451.

Narated By Abu Al-Bakhtari : I asked Ibn Umar about Salam (the fruits of) date-palms. He replied, “The Prophet 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 forbade the sale of dates till they were fit for eating and could be estimated.”