Sunan Ibn Majah, Chapter: The Chapters on Business Transactions

"I used to sell camels, and I used to buy gold for silver and silver for gold, Dinar for Dirham and Dirham for Dinar. I asked the Prophet (saw) about that, and he said: 'If you take one of them and give the other, then you and your companion should not separate until everything is clear (i.e., the exchange is completed).'" (Hasan) Another chain with similar wording.