عَنِ ابْنِ عُمَرَ رَضِيَ اللَّهُ عَنْهُمَا قَالَ سَأَلْتُ النَّبِيَّ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ أَشْتَرِي الذَّهَبَ بِالْفِضَّةِ أَوِ الْفِضَّةَ بِالذَّهَبِ قَالَ إِذَا أَخَذْتَ وَاحِدًا مِنْهُمَا بِالْآخَرِ فَلَا يُفَارِقُكَ صَاحِبُكَ وَبَيْنَكَ وَبَيْنَهُ لَبْسٌ
It is narrated from Sayyiduna Abdullah bin Umar, may Allah be pleased with them both, that he said: I asked the Noble Prophet, peace and blessings be upon him, whether I could buy gold in exchange for silver or silver in exchange for gold. The Prophet, peace and blessings be upon him, said: When you purchase one thing in exchange for another, your companion should not part from you while there remains anything related to that transaction between you and him.