Sahih al-Bukhari 2249, 2250

Muhammad ibn Bashshar narrated to us: Ghundar narrated to us: Shuba narrated to us, from Amr, from Abu al-Bakhtari, who said: I asked Ibn Umar, may Allah be pleased with them both, about advance sales for dates. Ibn Umar replied: “The Prophet forbade selling produce until it ripened, and forbade silver for gold on deferred delivery in exchange for immediate payment.” I also asked Ibn Abbas. He replied: “The Prophet forbade selling dates until they could be eaten from and weighed.” I asked him, “What is to be weighed?” A man beside Ibn Abbas said: “Until they have been harvested and secured.”