Yahya related to me from Malik that Abu'z-Zinad heard Said ibnal-Musayyab say, "There is usury only in gold or silver or what isweighed or measured of what is eaten or drunk." (DE)
USC-MSA web (English) reference: Book 31, Hadith 37 (DE)
Arabic reference (DE): Book 31, Hadith 1328 (DE)