It is narrated from Sayyiduna Ubadah bin Samit (may Allah be pleased with him) that the Prophet Muhammad (peace be upon him) said: "Do not sell gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, or salt for salt except in equal amounts, hand to hand. But if it is gold for silver, silver for gold, wheat for barley, barley for wheat, dates for salt, or salt for dates, then sell as you wish (equality in weight is not necessary), but it must be hand to hand. If the commodity is the same, then any increase or decrease is riba (usury)."
Hadith Referenceالسنن المأثورة / باب في البيوع / 218
Hadith Takhrijسنن نسائی، البيوع، باب بيع البر بالبر، رقم : 4560، سنن ابن ماجه التجارات ، باب الصرف و مالا يجوز متفاضلا يداً بيد ، رقم : 2254، وقال الالباني: صحيح ۔