Maalik ibn Aws ibn Hadathan (RA) narrated: I advanced (into the market) saying, “Who will exchange my dinars for dirhams?” Talhah ibn Ubaydullah who was sitting by Umar ibn Khattab said (to me), “Show me your gold and come back (after a while) when my servant comes and I will pay you by silver.” Umar ibn Khattab exclaimed, “Certainly not! By Allah, you will pay him the dirham (now) or return to him his gold. For, Allah’s Messenger said, ‘Silver for gold is interest unless it is prompt and on spot (payment), and wheat for wheat is interest unless it is prompt and on spot (payment), and barley for barley is interest unless it is prompt and spot (payment), and dates for dates is interest unless it is prompt and spot (payment)’.”
[Bukhari 1081, Muslim 1586]