Sahih Bukhari Volume 3, Book 34, Hadith Number 342.

Narated By Tawus : Ibn ‘Abbas said, “Allah’s Apostle forbade the selling of foodstuff before its measuring and transferring into one’s possession.” I asked Ibn ‘Abbas, “How is that?” Ibn ‘Abbas replied, “It will be just like selling money for money, as the foodstuff has not been handed over to the first purchaser who is the present seller.”