Abu al-Ash'ath was present at the sermon of Sayyiduna Ubadah (may Allah be pleased with him), in which he narrated from the Prophet (peace and blessings be upon him), saying: "Gold for gold, equal for equal, and silver for silver, equal for equal in weight, and wheat for wheat, equal in measure, and barley for barley, equal in measure, and dates for dates, and salt for salt. Whoever gives more or asks for more has taken usury (riba)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10541