Musnad Ahmad 31:476

Rawh narrated to us, Sulayman ibn Ali narrated to us, Abu al-Mutawakkil al-Naji narrated to us, Abu Sa’id al-Khudri narrated from the Prophet, may God bless him and grant him peace. A man from the people said to him: "Is there anyone between you and the Prophet, may God bless him and grant him peace, other than Abu Sa’id?" He said: "No, by God, there is no one between me and the Prophet, may God bless him and grant him peace, other than Abu Sa’id." He said: "Gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt, equal for equal. Whoever increases or seeks an increase, he has committed usury; the taker and the giver in it are equal."