Abu al-Ash'ath al-San'ani (may Allah have mercy on him) said that he was present at the sermon of Ubadah bin al-Samit (may Allah be pleased with him), in which he was narrating a hadith from the Prophet (peace and blessings be upon him): "There is no harm in selling barley for wheat, even when the barley is more."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب البيوع / 10505