It is narrated from Abu Ash'ath San'ani (may Allah have mercy on him) that during the rule of our master Amir Muawiyah (may Allah be pleased with him), some people came to the markets who were selling gold vessels and necklaces, etc. Then our master Ubadah bin Samit (may Allah be pleased with him) stood up and said: The Messenger of Allah (peace and blessings be upon him) forbade selling gold for gold, silver for silver, wheat for wheat, dates for dates, barley for barley, and salt for salt except when both sides are equal and of the same weight. Whoever gives more or takes more has engaged in usury.
Hadith Referenceالسنن المأثورة / باب في البيوع / 221
Hadith Takhrijصحيح مسلم ، المساقاة، باب الصرف وبيع الذهب بالورق نقداً، رقم : 1587 ۔