Musnad Ahmad 31:566

Mu'tamir narrated to us, from 'Asim, from Shurahbil, that Ibn 'Umar, Abu Hurairah, and Abu Sa'id narrated that the Prophet, peace and blessings of Allah be upon him, said: "Gold for gold, like for like; and silver for silver, like for like, hand to hand (spot transaction). Whoever increases or seeks an increase, he has engaged in usury (riba)." Shurahbil said: "If I did not hear it, may Allah cast me into the Fire."