Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) said: Sell gold for gold and silver for silver equal for equal, and do not sell silver for gold in such a way that one is on credit and the other in cash. If he asks you for time until he enters his house, do not give him time; (deal) only hand to hand. Indeed, I fear usury (rama') for you.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10511