Musnad Ahmad 31:644

Yazid narrated to us, Sulayman ibn Ali 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, dates for dates, wheat for wheat, barley for barley, and salt for salt, equal for equal, like for like. Whoever increases or seeks an increase has committed usury (riba); the taker and the giver are the same."