Sahih Muslim, Hadith 1586a

I came saying who was prepared toexchange dirhams (for my gold), whereupon Talha b. Ubaidullah (Allah be pleased with him) (as he was sitting with 'Umar b. Khattib) said: Show us your gold and then come to us (at a later time). When our servant would come we would give you your silver (dirhams due to you). Thereupon 'Umar b. al-Khattib (Allah be pleased with him) said: Not at all. By Allah, either give him his silver (coins). or return his gold to him, for Allah's Messenger (ﷺ) said: Exchange of silver for gold (has an element of) interest in it. except when (it is exchanged) on the spot;and wheat for wheat is an interest unless both are handed over on the spot: barley for barley is interest unless both are handed over on the spot; dates for dates is interest unless both are handed over on the Spot.