Al-Muwatta Hadith 31.38

Yahya related to me from Malik from Ibn Shihab from Malik ibn Aus ibn al-Hadathan an-Nasri that one time he asked to exchange 100 dinars. He said, "Talha ibn Ubaydullah called me over and we made a mutual agreement that he would make an exchange for me. He took the gold and turned it about in his hand, and then said, 'I can't do it until my treasurer brings the money to me from al-Ghaba.' Umar ibn al-Khattab was listening and Umar said, 'By Allah! Do not leave him until you have taken it from him!' Then he said, 'The Messenger of Allah, may Allah bless him and grant him peace, said, "Gold for silver is usury except hand to hand. Wheat for wheat is usury except hand to hand. Dates for dates is usury except hand to hand. Barley for barley is usury except hand to hand." "' Malik said, "When a man buys dirhams with dinars and then finds a bad dirham among them and wants to return it, the exchange of the dinars breaks down, and he returns the silver and takes back his dinars. The explanation of what is disapproved of in that is that the Messenger of Allah, may Allah bless him and grant him peace, said, 'Gold for silver is usury except hand to hand.' and Umar ibn al-Khattab said, 'If someone asks you to wait to be paid until he has gone back to his house, do not leave him.' When he returns a dirham to him from the exchange after he has left him, it is like a debt or something deferred. For that reason, it is disapproved of, and the exchange collapses. Umar ibn al-Khattab wanted that all gold, silver and food should not be sold for goods to be paid later. He did not want there to be any delay or deferment in any such sale, whether it involved one commodity or different sorts of commodities."