Maliks Muwatta Book 31, Hadith Number 34.

Yahya related to me from Malik from Nafi from Abdullah ibn Umar that Umar ibn al-Khattab said, “Do not sell gold for gold except like for like, and do not increase one part over another part. Do not sell silver for silver except like for like, and do not increase one part over another part. Do not sell silver for gold, one of them at hand and the other to be given later. If someone seeks to delay paying you until he has been to his house, do not leave him. I fear rama for you.” Rama is usury.