Sahih Bukhari, Chapter: Sales and Trade

from Malik bin Aus, that the latter said, "Who has change?" Talha said, "I (will have change) when our store-keeper comes from the forest." Narrated Umar bin Al-Khattab: Allah's Apostle said, "The bartering of gold for silver 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." (See Riba-Fadl in the glossary).