Sahih al-Bukhari Book 34, Hadith 85
The Prophet (ﷺ) said, "He who buys foodstuff should not sell it till he has received it."
Ibn `Abbas said, "Allah's Messenger (ﷺ) 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."
That the latter said, "Who has change?" Talha said, "I (will have change) when our storekeeper comes from the forest."
Malik bin Aus narrated from `Umar bin Al-Khattab: Allah's Messenger (ﷺ) said, "The bartering of gold for gold is Riba (usury), except if it is from hand to hand and equal in amount, and wheat grain for wheat grain is usury except if it is form hand to hand and equal in amount, and dates for dates is usury except if it is from hand to hand and equal in amount, and barley for barley is usury except if it is from hand to hand and equal in amount."