The Prophet of Allah (sws) is reported to have said:¹  

If you sell gold on credit, take back gold of the same type and the same quantity; and if you sell silver on credit, take back silver of the same type and the same quantity; for he who gave more or desired more, then this is precisely what Riba is. (Muslim, Kitabu’l Buyu‘) 
  
If you sell silver on credit in exchange for gold there is the possibility of Riba in it.² Similarly wheat in exchange for wheat of another type, barley in exchange for barley of another type, dates for dates of another type.³ However, if the exchange is done on the spot, then there is no harm in it. (Muslim, Kitabu’l Buyu‘)

   These directives are meant to prevent Riba from creeping into barter on credit through the back door. It is obvious from these directives that the Prophet (sws) wanted his followers to refrain even from the traces of Riba.

^1. In some versions of these Ahadith, the words ‘on the spot’, or words to that effect, the second Hadith given above have been erroneously included in the first Hadith. Similarly, the words ‘gold for gold’ (of the same quantity and type) in the first Hadith quoted above have been erroneously put in place of the words ‘silver in exchange for gold’. The confusion of Riba al-Fadl emerged in our religious literature owing to this hotchpotch. Actually, as the two Ahadithquoted above show, the correct picture is this regard is what is depicted in the following words of the Prophet (sws): ‘Riba is only in lending’ (Muslim, Kitabu’l Buyu‘) 
^2. When heterogeneous goods are exchanged on credit, the possibility of increase on this ‘lending’ means that there is a possibility of Riba creeping in. 
^3. The copulative relation of the following sentences with the sentence, ‘If you sell silver [on credit] in exchange for gold’, in which sentence two different commodities are mentioned (gold and silver), entails that the commodities mentioned in the following sentences be taken as heterogeneous, that is wheat for wheat of another type, etc.