Sahih Bukhari Volume 3, Book 34, Hadith Number 386.

Narated By Abu Salih Az-Zaiyat : I heard Abu Said Al-Khudri saying, “The selling of a Dinar for a Dinar, and a Dirham for a Dirham (is permissible).” I said to him, “Ibn ‘Abbas does not say the same.” Abu Said replied, “I asked Ibn ‘Abbas whether he had heard it from the Prophet s or seen it in the Holy Book. Ibn ‘Abbas replied, “I do not claim that, and you know Allah’s Apostle better than I, but Usama informed me that the Prophet had said, ‘There is no Riba (in money exchange) except when it is not done from hand to hand (i.e. when there is delay in payment).'”