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." (IT)
USC-MSA web (English) reference: Book 31, Hadith 37 (IT)
Arabic reference (IT): Book 31, Hadith 1328 (IT)