Musnad Ahmad 31:927

Waki' narrated to us, Isma'il bin Muslim al-'Abdi narrated to us, Abu al-Mutawakkil al-Naji narrated to us, from Abu Sa'id al-Khudri, 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, dates for dates, and salt for salt, like for like, hand to hand. Whoever increases or seeks an increase has engaged in usury (riba); the taker and the giver in it are equal."