Musannaf Ibn Abi Shayba 13:645

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Sufyan narrated to us, from Khalid, from Abu Qilabah, from Abu al-Ash’ath al-San’ani, from Ubadah ibn al-Samit, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Gold for gold, silver for silver, wheat for wheat, and barley for barley, like for like, hand to hand. But if these categories differ, then sell as you wish, provided it is hand to hand."