Musannaf Ibn Abi Shayba 13:2533

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, from ‘Ubadah ibn al-Samit, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: Gold for gold, silver for silver, wheat for wheat, barley for barley, and salt for salt, like for like, hand to hand. But if these categories differ, then sell as you wish, provided it is hand to hand.