Musnad Ahmad 1012:62

Waki’ narrated to us, from Sufyan, from Khalid al-Hadhdha’, from Abu Qilabah, from Abu al-Ash’ath al-San’ani, from ‘Ubadah ibn al-Samit, who said: The Messenger of Allah (ﷺ) said: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt, like for like, hand to hand. But if these types differ, then sell as you wish, if it is hand to hand."