Yahya related to me from Malik from Nafi from Abdullah ibn Umarthat Umar ibn al-Khattab said, "Do not sell gold for gold except likefor like, and do not increase one part over another part. Do not sellsilver for silver except like for like, and do not increase one partover another part. Do not sell silver for gold, one of them at handand the other to be given later. If someone seeks to delay paying youuntil he has been to his house, do not leave him. I fear rama foryou." Rama is usury.
USC-MSA web (English) reference: Book 31, Hadith 34
Arabic reference: Book 31, Hadith 1325