Musnad al-Humaydi

Sayyiduna Ubadah bin Samit (may Allah be pleased with him) narrates: The Noble Prophet (peace and blessings be upon him) said: “The exchange of gold for gold will be equal for equal. The exchange of silver for silver will be equal for equal. The exchange of dates for dates will be equal for equal. The exchange of wheat for wheat will be equal for equal. The exchange of barley for barley will be equal for equal, until the Noble Prophet (peace and blessings be upon him) specifically mentioned the exchange of salt for salt as well (and said): Whoever gives more or demands more, then this is usury.”