Sahih Al Bukhari Hadith Number 2134

Narrated Az-Zuhri from Malik bin Aus: 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 (peace be upon him) 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.”