The Prophet said, The selling of wheat for wheat is Riba (usury) ...

Hadith 3:379

Narrated Ibn 'Umar:

The Prophet said, "The selling of wheat for wheat is Riba (usury) except if it is handed from hand to hand and equal in amount. Similarly the selling of barley for barley, is Riba except if it is from 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. (See Riba-Fadl in the glossary).