Al-Zuhri (may Allah have mercy on him) said that Sa'id bin al-Musayyib (may Allah have mercy on him) said: Usury is only in gold and silver, or in things that are measured or weighed and that relate to eating and drinking.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10521