Musnad Abu Ya'la al-Mawsili 42:352

Zuhayr narrated to us, Jarir narrated to us, from Mughirah, from Abd al-Rahman ibn Abi Nu'm, who said: Abu Sa'id al-Khudri came to a man and said to him, "Have you read what we have not read, and have you accompanied [the Prophet] as we have accompanied [him]?" The man said, "I have not read except what you have read, and you have accompanied [him]." He [Abu Sa'id] said, "Then why do you issue fatwas to the people [allowing] two dirhams for three, and a dirham for two dirhams?" Abu Sa'id said, "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'Gold for gold, like for like; whatever is in excess is usury. And silver for silver, like for like; whatever is in excess is usury.'" He said, "I heard him afterwards saying: 'O Allah, I repent to You for what I used to issue fatwas to the people regarding in currency exchange.'"