Sahih al-Bukhari Book 34, Hadith 87
The Prophet (ﷺ) forbade the selling of foodstuff before receiving it. I consider that all types of sellings
should be done similarly.
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."
The Prophet (ﷺ) said, "The buyer of foodstuff should not sell it before it has been measured for him." Isma`il narrated instead, "He should not sell it before receiving it."