Imam Malik's Muwatta, Chapter: Business Transactions

Yahya related to me from Malik from Abdullah ibn Dinar from Abdullah ibn Umar that Umar ibn al-Khattab said, "Do not sell gold for gold except like for like. Do not increase part of it over another part. Do not sell silver for silver except like for like, and do not increase part of it over another part. Do not sell some of it which is there for some of it which is not. If someone asks you to wait for payment until he has been to his house, do not leave him. I fear rama for you." Rama is usury.