Narrated Ibn Shihab: that Malik bin Aus said, "I was in need of change for one-hundred Dinars. Talha bin 'Ubaid-Ullah called me and we discussed the matter, and he agreed to change (my Dinars). He took the gold pieces in his hands and fidgeted with them, and then said, "Wait till my storekeeper comes from the forest." 'Umar was listening to that and said, "By Allah! You should not separate from Talha till you get the money from him, for Allah's Apostle said, 'The selling of gold for gold is Riba (usury) except if the exchange is from hand to hand and equal in amount, and similarly, the selling of wheat for wheat is Riba (usury) unless it is from hand to hand and equal in amount, and the selling of barley for barley is usury unless it is from hand to hand and equal in amount, and dates for dates, is usury unless it is from hand to hand and equal in amount"

— Sahih Bukhari, Volume 3, Book 34: Sales and Trade, Hadith #382