Musnad Ahmad 238
Malik bin Aws bin al-Hadathan said:
I exchanged some silver for gold with Talhah bin ‘Ubaidullah, and he said: Wait for me until our storekeeper comes from al-Ghabah, `Umar bin al-Khattab heard that and said: No, by Allah! Do not leave him until you take your exchange from him, for I heard the Messenger of  Allah (ﷺ) say: `Gold for silver is riba unless it is `here, take this` and `here, take this` [i.e., exchanged on the spot].”
حَدَّثَنَا عَبْدُ الرَّزَّاقِ، أَنْبَأَنَا مَعْمَرٌ، عَنِ الزُّهْرِيِّ، أَخْبَرَنِي مَالِكُ بْنُ أَوْسِ بْنِ الْحَدَثَانِ، قَالَ صَرَفْتُ عِنْدَ طَلْحَةَ بْنِ عُبَيْدِ اللَّهِ وَرِقًا بِذَهَبٍ فَقَالَ أَنْظِرْنِي حَتَّى يَأْتِيَنَا خَازِنُنَا مِنْ الْغَابَةِ قَالَ فَسَمِعَهَا عُمَرُ بْنُ الْخَطَّابِ رَضِيَ اللَّهُ عَنْهُ فَقَالَ لَا وَاللَّهِ لَا تُفَارِقُهُ حَتَّى تَسْتَوْفِيَ مِنْهُ صَرْفَهُ فَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يَقُولُ الذَّهَبُ بِالْوَرِقِ رِبًا إِلَّا هَاءَ وَهَاءَ‏.‏