Sayyiduna Abu Sa'id (may Allah be pleased with him) said: Some dates were brought to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) asked: "Are these from our dates?" A man said: O Messenger of Allah! We take one sa' of these in exchange for two sa' of our dates. He (peace and blessings be upon him) said: "This is usury (riba). Return it, and sell our dates, then buy these dates for us."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10567